Please ensure that you are using the latest release of RadControls for ASP.NET AJAX (Q1 2013 BETA). Also, I would suggest checking if the custom CSS styles that you have applied to the dock are overriding its default styles, which results in the examined issue. You can do so by removing the custom styles.
If the problem persists please open the online demos of RadDock
and provide step by step instructions for reproducing it in any of the examples so that I can inspect the issue locally and provide an according solution.
the Telerik team
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 their blog feed