Please refer to the following help topic
which elaborates on adding ajax functionality in SP. Ensure that you are using the correct approach for achieving the required functionality.
Also please test the application by using regular asp UpdatePanel instead of RadAjax and verify of the ajax request works as expected in this case.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed