Note that you could not add control which is initially invisible on the page, to the RadAjaxManager settings. The initiator or the updated control should be always placed in a visible container so that the manager can locate it when necessary.
Please review the help topic below for more information on this matter:
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