is this related to your previous post? :
Working with UI Virtualization can be a bit tricky. You have to make sure that an Element exists before you select it. What this means is that you have to make sure you've correctly recorded the action which causes the desired item to be loaded. This action will usually be a scroll-down of the checkbox item. Since this scroll-down isn't recorded your test will fail. This happen because when you execute the test your item doesn't exists when you try to select it.
You might have to use "Mouse Actions" to simulate real mouse clicks in order to make sure the "load" event for your item is fired. Check out the section on "Mouse Actions" in our User's Guide:
Also, to repeat the previous e-mail:
Would it be possible for you to provide us either direct access to your application or with the actual code of the application you are trying to handle or a small sample Silverlight application showing the ComboBox you are trying to work with? We will also provide you with fully working code which you will be able to use straight away.
Also consider letting us look at the application on your PC. Would it be possible to arrange a GoToMeeting in order for us to check out your app directly on your PC via screen sharing?
the Telerik team