Progress bar in Ajax Update panel not showing

I wrote the following

 <asp:UpdatePanel ID="Download" runat="server">
     <ContentTemplate>
         <asp:Label ID="lblMessage"  runat="server" 
           Text="Click the link to download the Report:"></asp:Label>  
         <asp:LinkButton ID="lbtnDownload" runat="server"  
           OnClick="lbtnDownload_C开发者_如何学编程lick" Text="Download Excel Sheet"></asp:LinkButton> 
     </ContentTemplate>
     <Triggers>
         <asp:PostBackTrigger ControlID="lbtnDownload"/>
     </Triggers>
 </asp:UpdatePanel>

 <asp:UpdateProgress ID="download1" runat="server" AssociatedUpdatePanelID="Download">
     <ProgressTemplate> 
         <asp:Image ID="Image1" runat="server" ImageUrl"indicator.gif" " />  
     </ProgressTemplate>
 </asp:UpdateProgress>

But the progress bar image is not showing can any one tell why

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Remove the following: <asp:PostBackTrigger ControlID="lbtnDownload"/>

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Ok, first thing to check is that you are definitely posting back successfully, also add a delay interval:

<asp:UpdateProgress DisplayAfter="0" ID="download1" runat="server"
   AssociatedUpdatePanelID="Download">