I'm wondering if you could post and .aspx + .ascx sample demonstrating how to reproduce this problem?
Do you have a <ext:ScriptManager> control on the parent .aspx Page?
There's also the <ext:ScriptManagerProxy> control which you could add to your .ascx UserControl, although if you're just adding simple control markup the Proxy should not be required.
Anyways, a bit more information should help us diagnose further.
Please let us know if you have already found the solution and/or post a note re: what was required to solve the problem.