Beginrequest and endrequest are not fired when postbacktrigger is added to updatepanel

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ASP.NET

Javascript

I have two asp buttons in the same update panel; For one of them I am using

<asp:PostBackTrigger ControlID="btnVerify" />

for the other I am not. But for both a progress bar should be fired; To fire this progress bar I am using begin and end request in a script block as below:

<script language="javascript" type="text/javascript">

    Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(InitializeRequest);
    Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequest); 

    function InitializeRequest(sender, args) {
         // alert("Start");
         var mdlPopUp = $find('<%= mdlPopup.ClientID %>');
         mdlPopUp.show();
    }

    function EndRequest(sender, args) {
         // alert("End");
         var mdlPopUp = $find('<%= mdlPopup.ClientID %>');
         mdlPopUp.hide();
    }

  </script>

mdlPopup is the progress bar.

Below is my UpdatePanel:

<asp:UpdatePanel ID="UPAll" runat="server">
   <ContentTemplate>

      <asp:LinkButton ID="lnkBtn" runat="server">
         <asp:Image ID="Image2" runat="server" ImageUrl="~/images/BAL/go.gif" />
      </asp:LinkButton>

      <asp:ImageButton ID="btnVerify" TabIndex="14" runat="server" ImageUrl="~/Images/Misc/verify.gif">
      </asp:ImageButton>

   </ContentTemplate>
   <Triggers>
      <asp:PostBackTrigger ControlID="btnVerify" />
   </Triggers>
</asp:UpdatePanel>

Now the problem is that whenever I press on btnVerify progress bar is not shown; BUT if I press on lnkBtn progress bar is shown. I know that the problem is from postbackTrigger (I removed it from triggers and progress bar was displayed as excpected, but then, btnVerify won't work correctly). So how can I resolve this issue and display the progress bar when I press on both buttons?

Update: Please note that I press on btnVerify: InitializeRequest and EndRequest are not fired (start and end are not alerted; test init is alerted)

What I have tried:

I have made some searches and I found that I should add this in javascript:

Sys.Application.add_init(appl_init);

function appl_init() {
  // alert("test init"); 
   Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(InitializeRequest);     
   Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequest);            
}

But unfortunately this did not help me, the progress bar is not fired when I press on btnVerify.