How to solve Trigger Event not fired button

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C#

ASP.NET

update Panel using in html page in web application so Button using trigger but not fired yes button.
Yes Button Click means not fired this is my problem please help me.

<asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Always" ChildrenAsTriggers="true">
    <ContentTemplate>
    <asp:Panel ID="pn2" runat="server" Visible = "true"  BackColor = "Bisque" Width = "770px">
                <table id="tblPopup" runat="server">
                                <tr>

                                <td>
                                   <asp:Label ID="lblEmployeeREjected" runat="server"   Text="are u sure want to delete current employee. Press Yes to Delete and No to cancel"
                                    Font-Bold="True" ForeColor="#3E3EFF" Width="770px" BackColor="White" Visible="false" ></asp:Label>

                                </td>
                                </tr>

                                <tr>
                                   <td align = "center">
                                      <asp:Button ID="btnYes" runat="server" OnClick="btnYes_Click" Text="Yes"  CausesValidation="false"
                                Width="50px"  />
                                <asp:Button ID="btnNo" runat="server" OnClick="btnNo_Click" Text="No"  CausesValidation="false"
                                 Width="50px" />
                                   </td>
                                </tr>
               </table>
                </asp:Panel>
      <asp:ModalPopupExtender ID="MPE" runat="server" PopupControlID="pn2" TargetControlID="tblPopup" PopupDragHandleControlID="header" BehaviorID="btnYes"
 BackgroundCssClass="Bg">
            </asp:ModalPopupExtender>
    </ContentTemplate>
    <Triggers>
    <asp:PostBackTrigger ControlID="btnYes" />
    <%--<asp:PostBackTrigger ControlID="btnYes" />
<asp:PostBackTrigger ControlID="btnNo" />--%>
<%--<asp:AsyncPostBackTrigger ControlID="btnYes" />
--%>    </Triggers>
    </asp:UpdatePanel>