Could you please give me some additional info, any of the following will make it that much easier to analyze and resolve the issue:
-Can you find a demo which demonstrates the issue either on our Silverlight demos page (http://demos.telerik.com/silverlight/#ComboBox
-Could you please provide access to your application that exhibits this behavior?
-Could you please get a screenshot of your test or just send the test iteself?
-Could you please record a video demonstrating the issue (Jing is really good)
The more info you give us the easier it is for us to assist you effectively.
the Telerik team