Event coud'nt fire on button click which is in modalpop control

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ASP.NET

I have taken modalpop and want to fire event on cinfirm button in modalpoup but doesnt go into that event...

//code

<asp:Button ID="BtnATcart" runat="server" Text="Add To Cart"
         onclick="BtnATcart_Click" />

         <cc1:ConfirmButtonExtender ID="ConfirmButtonExtender1" runat="server"  TargetControlID="BtnATcart" DisplayModalPopupID="ModalPopupExtender1" >
     </cc1:ConfirmButtonExtender>

     <cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="BtnATcart" PopupControlID="Panel1" OkControlID="BtnAT_confirm" CancelControlID="BtnAT_Cancel">
     </cc1:ModalPopupExtender>


    <asp:Panel ID="Panel1" runat="server" Style="display: none; width: 250px; background-color:Silver;
   border-width: 2px; border-color: Black; border-style: solid; padding: 20px;">
      Are you sure you ?
      <br /> <br />
        <asp:Label ID="LblAT_quantity" runat="server" Text="Quantity"></asp:Label>
        <asp:TextBox ID="TxtAT_quantity" runat="server"></asp:TextBox>
        <br /> <br />
        
        <br />
        <asp:CheckBox ID="ChkboxAT_iagree" runat="server" />
        I Agree
     <br />
           

        <asp:Button ID="BtnAT_confirm" runat="server" Text="Confirm" Width="60px"
            Height="25px"  style="margin-top:40px;" onclick="BtnAT_confirm_Click" />
           <asp:Button ID="BtnAT_Cancel" runat="server" Text="Cancel" Width="60px"
            Height="25px" onclick="BtnAT_Cancel_Click" />
       </asp:Panel>