An educated guess: this happens inside an update panel, doesn't it?
This happened to me once, the reason that no error/exception is thrown is that nothing actually is wrong in the back end. Your file data goes back to the front-end, which rejects it to render the content of the update panel.
If that's your case, you have to put the control who fires the postback (i.e. btnDownload
)as a sync trigger of the update panel:
<Triggers>
<asp:PostBackTrigger ControlID="gdvSongs" />
</Triggers>