Dropdownlist is not loading data inside the udpdatepanel

<asp:UpdatePanel ID="pnlRating" runat="server">
<Triggers >
<asp:AsyncPostBackTrigger ControlID="ratingControl" EventName="Changed" />

</Triggers>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="DdlStore" EventName="SelectedIndexChanged" />
</Triggers>
<ContentTemplate>

<table width="50%">
<tr>
<td class="style1">
    Select StoreId</td>
<td>
    <asp:DropDownList ID="DdlStore" EnableViewState="true" runat="server" AutoPostBack="True"
        Height="17px" Width="152px">
    </asp:DropDownList>
    </td>
</tr>
    <tr>
        <td class="style1">
            <b>Average Rating:</b>
        </td>
        <td>
            <asp:Rating ID="ratingControl" runat="server" AutoPostBack="true"
                EmptyStarCssClass="ratingEmpty" FilledStarCssClass="ratingFilled"
                OnChanged="RatingControlChanged" StarCssClass="ratingEmpty"
                WaitingStarCssClass="ratingSaved">
            </asp:Rating>
            <b>
            <asp:Label ID="lbltxt" runat="server" />
            </b>
        </td>
    </tr>
    <tr>
        <td class="style1">
            &nbsp;</td>
        <td>
            &nbsp;</td>
    </tr>
    <tr>
        <td class="style1">
            &nbsp;</td>
        <td>
            <asp:Button ID="BtnSend" runat="server" onclick="BtnSend_Click" Text="Send"
                BackColor="#996633" BorderColor="#CC3300" BorderStyle="Solid" Width="150px" />
        </td>
    </tr>
</table>
</ContentTemplate>
</asp:UpdatePanel>