I'm afraid it is difficult to pinpoint the exact reason without reproducing the actual problem locally. Please temporarily disable any AJAX on the page if present (RadAjaxManager, RadAjaxPanel, UpdatePanel, etc.) and enable your script debugger (FireBug or F12) to see whether there are any script or server errors interfering, and make sure that the application works without AJAX.
If the problem persists, please modify the attached dummy web site sample to reproduce the problematic behavior and open a formal support thread to send it back to us. This will enable us to further investigate the matter and try to determine the responsible culprit.