The easiest way to find if the problem is AJAX-related is to disable the AJAX temporary. You can do that trough the EnableAJAX
property of the RadAjax control in question. If the context menu functions as expected this way, it's a matter of AJAX settings which needs to be properly configured. You may just need to set the menu as updated control so that the tab click will update it as well.
Let us know if you are able to isolate and resolve the problem or if you need further assistance.
All the best,
the Telerik team