Please, upgrade to the latest version of the control as the behavior you describe is implemented in RadDock Prometheus. Also, please check whether you have not by any chance set the RadDockZone property FitDocks - by default it is true and it is used exactly to achieve the behavior you would like to see. If you have set it to False, then docks won't fit in the zones, so remove it. If you continue to experience the same problem, please send us a small working project that demonstrates the wrong behavior and we will work on it.
the Telerik team