I am not really sure what might be causing this. I tried with a sample project, but everything was working fine at my side. I have attached the project so you can take a look and tell us if you are doing something different. It will be best if you could send us a sample project or a code snippet that can help us reproduce the issue and provide a solution.
You can also try to set the focus to the button in DIspatcher.
All the best,
the Telerik team