UPDATE PANEL PARTIAL POSTBACK IS NOT WORKING

<asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate>
            <table style="text-align: center" width="960px" runat="server" id="accepttbl">

        <tr title="Reviewer's comment" style="width: 960px" id="txtboxreview" runat="server">
            <td>
                <asp:Label ID="Labelreview" runat="server" Text="Reviewes / Comments : "></asp:Label>
            </td>

            <td colspan="2">
                <asp:TextBox ID="txtReview" runat="server" Width="425px" TextMode="MultiLine" Style="width: 800px"></asp:TextBox>
            </td>
        </tr>
        <tr style="width:960px;">
            <td></td>
             <td colspan="2">
                <asp:TextBox ID="txtrevisionmail" runat="server" Visible="false" Width="425px" Rows="7" TextMode="MultiLine" Style="width: 800px" placeholder="Please write revision mail for the principal investigator"></asp:TextBox>

            </td>
        </tr>
        <tr style="width: 960px" runat="server" id="reviewaccept">
            <td>
                <asp:Label ID="Label2" runat="server" Text="Accepted ? "></asp:Label>
            </td>
            <td>
               
                <asp:DropDownList ID="ddlStatus" runat="server" OnSelectedIndexChanged="ddlStatus_SelectedIndexChanged" AutoPostBack="true">
                    <asp:ListItem Value="0">NO</asp:ListItem>
                    <asp:ListItem Value="1" Selected="True">YES</asp:ListItem>
                </asp:DropDownList>
                  
            </td>
           
            <td colspan="2">
                <asp:Button ID="btnSubmitRevCom" runat="server" Text="Submit comment" CssClass="btn btn-primary" OnClick="btnSubmitRevCom_Click" />
            </td>
        </tr>
        <tr runat="server" id="officeremarks" visible="false">
            <td>
                <asp:TextBox ID="txtoffcremark" Width="900px" runat="server" TextMode="MultiLine" Rows="5" placeholder="Office Remarks"></asp:TextBox>

                <asp:Button ID="btnoffcremark" runat="server" Text="Submit Remarks" CssClass="btn btn-primary" OnClick="btnoffcremark_Click" />

            </td>
        </tr>
    </table>
      
    </ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="ddlStatus" EventName="SelectedIndexChanged"/>
        </Triggers>
    </asp:UpdatePanel>

protected void ddlStatus_SelectedIndexChanged(object sender, EventArgs e)
    {
        User loginUser = Session["LoginUser"] as User;
        ModuleSP msp = new ModuleSP();
        if (msp.HasAdminPrivilage(loginUser.Id, IntranetModules.EgrantAdmin))
        {
            if (loginUser.EmailId != "deanacad")
            {
                if (ddlStatus.SelectedValue == "0")
                {

                    txtrevisionmail.Visible = true;
                    //Email typing box display email goes when submit button press
                }
                else
                {
                    txtrevisionmail.Visible = false;
                }
            }
        }}