Asp dropdownlist not keeping value in modal dialog box

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ASP.NET4

C#6.0

I have a modal dialog box which contains several textboxes and a couple of dropdownlists.

The dialog is used to add a user to the system and works fine when adding just one user at a time. If I click the button to display the modal dialog a second time the dropdown boxes dont retain their values.

I have put the modal dialog inside an update panel and also make sure that in the selectedindexchanged methods that the modal dialog remains open.

In the page load I check to make sure the dropdowns are not populated on postback

If I go to another page on the website and then return to the page with the modal dialog the dropdownlists work fine and keep their values.

Any help would be much appreciated.

What I have tried:

<asp:Panel ID="pnlAddNewUser" runat="server" CssClass="modal fade">
    <div class="modal-dialog">
        <div class="modal-content">
            <asp:UpdatePanel ID="upPnlNewUser" runat="server" UpdateMode="Conditional">
            <ContentTemplate>
            <div class="modal-body">
                <div class="row">
                    <div class="col-md-12">
                        <fieldset>
                            <legend class="text-center header">Add New User</legend>

                            <asp:Panel ID="pnlCheckAD" runat="server">
                                <div class="form-group">
                                    <div class="col-md-3 text-right">
                                        <asp:Label ID="lblCheckAD" Text="Enter Name:" runat="server" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <asp:TextBox ID="txtCheckAD" placeholder="Enter username e.g. bloggsj" runat="server" class="form-control" ></asp:TextBox>
                                        <asp:Label ID="lblFindUserError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="col-md-3">
                                        <asp:Button ID="btnCheckAD" runat="server" Text="Check AD" OnClick="btnCheckAD_Click" CssClass="btn btn-success" />
                                        <asp:Button ID="btnCancelCheckAD" Text="Cancel" runat="server" CssClass="btn btn-primary" OnClientClick="HideAddModal(); return false;" />
                                        <div class="clearfix"></div>
                                    </div>
                                </div>
                                </asp:Panel>

                            <asp:Panel ID="pnlAddUser" runat="server" Visible="false" CssClass="modal-fade">

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblFirstName" Text="First Name:" runat="server" class="control-label col-md-3 text-right" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <asp:TextBox ID="txtFirstName" placeholder="Enter first name" runat="server" class="form-control" ></asp:TextBox>
                                        <asp:Label ID="lblFirstNameError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblLastName" Text="Last Name:" runat="server" class="control-label col-md-3 text-right" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <asp:TextBox ID="txtLastName" placeholder="Enter last name" runat="server" class="form-control" ></asp:TextBox>
                                        <asp:Label ID="lblLastNameError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblEmail" Text="Email:" runat="server" class="control-label col-md-3 text-right" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <asp:TextBox ID="txtEmail" placeholder="Enter email address" runat="server" class="form-control" ></asp:TextBox>
                                        <asp:Label ID="lblEmailError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblPhone" Text="Phone Number:" runat="server" class="control-label col-md-3 text-right" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <asp:TextBox ID="txtPhone" placeholder="Enter phone number" runat="server" class="form-control" ></asp:TextBox>
                                        <asp:Label ID="lblPhoneError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblMobile" Text="Mobile Number:" runat="server" class="control-label col-md-3 text-right" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <asp:TextBox ID="txtMobile" placeholder="Enter mobile number" runat="server" class="form-control" ></asp:TextBox>
                                        <asp:Label ID="lblMobileError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblJobTitle" Text="Job Title:" runat="server" class="control-label col-md-3 text-right" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <asp:TextBox ID="txtJobTitle" placeholder="Enter job title" runat="server" class="form-control" ></asp:TextBox>
                                        <asp:Label ID="lblJobTitleError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblUserName" Text="User Name:" runat="server" class="control-label col-md-3 text-right" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <asp:TextBox ID="txtUserName" placeholder="Enter user name" runat="server" class="form-control" ></asp:TextBox>
                                        <asp:Label ID="lblUserNameError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblPassword" Text="Password:" runat="server" class="control-label col-md-3 text-right" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <asp:TextBox ID="txtPassword" placeholder="Enter user password" runat="server" class="form-control" Enabled="true"></asp:TextBox>
                                        <asp:Label ID="lblPasswordError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblGroup" Text="Groups:" runat="server" class="control-label col-md-3 text-right" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <%--<asp:DropDownList ID="ddlGroups" runat="server" class="form-control"></asp:DropDownList>--%>
                                        <asp:ListBox ID="ddlGroups" runat="server" CssClass="form-control" SelectionMode="Multiple" Rows="6" OnSelectedIndexChanged="ddlGroups_SelectedIndexChanged" AutoPostBack="true"></asp:ListBox>
                                        <asp:Label ID="lblGroupError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblDepot" Text="Depots:" runat="server" class="control-label col-md-3 text-right" ></asp:Label>
                                    </div>
                                    <div class="col-md-6">
                                        <%--<asp:DropDownList ID="ddlDepots" runat="server" class="form-control" SelectMethod=""></asp:DropDownList>--%>
                                        <asp:ListBox ID="ddlDepots" runat="server" class="form-control" SelectionMode="Multiple" Rows="12" OnSelectedIndexChanged="ddlDepots_SelectedIndexChanged" AutoPostBack="true"></asp:ListBox>
                                        <asp:Label ID="lblDepotError" runat="server" class="text-danger"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-3">
                                        <asp:Label ID="lblInternalUser" runat="server" Text="Internal User"></asp:Label>
                                    </div>
                                    <div class="col-md-9">
                                        <asp:CheckBox ID="chkInternalUser" runat="server" />
                                    </div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-12 text-right">
                                        <asp:Button ID="btnAddUser" Text="Save" runat="server" CssClass="btn btn-success" OnClick="btnAddUser_Click"  />
                                        <asp:Button ID="btnCancelAdd" Text="Cancel" runat="server" CssClass="btn btn-primary" OnClientClick="HideAddModal(); return false;" />
                                        <asp:Button ID="btnCloseAdd" runat="server" Text="Close" OnClientClick="HideAddModal();" Visible="false" OnClick="btnCloseAdd_Click" CssClass="btn btn-success" />
                                    </div>
                                </div>

                                <div class="form-group">
                                    <div class="col-md-12 text-center">
                                        <asp:Label ID="lblAddSuccess" runat="server" CssClass="text-success" Visible="false"></asp:Label>
                                    </div>
                                    <div class="clearfix"></div>
                                </div>

                            </asp:Panel>
                        </fieldset>
                    </div>
                </div>
            </div> <%--modal-body--%>
           </ContentTemplate>
           </asp:UpdatePanel>
        </div>
    </div>
</asp:Panel>