Based on the provided information it is hard to say what is causing the described issue. Could you please post the full aspx page markup with the related code behind? Thus we will be able to gather more details about your scenario and provide you with more to-the-point answer.
Looking forward for your reply.
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 the blog feed