This might happen if the hosted control sets MouseButtonEventArgs.Handled = true in mouse button related events, or if it captures the mouse. That prevents the mouse events from reaching the diagram. In WpfDiagram are able to work around this by handling the preview version of the mouse events, which unfortunately are not available in Silverlight.

If you are using your own controls, try setting Handled = false from the mouse events, or don't capture the mouse. Otherwise you could try calling Diagram.RaiseClicked from a similar event raised for the hosted control, e.g. Button.Clicked.

I hope that helps,
Stoyan