WebControl.ContextualMenuAction() event no firing

I’m having trouble using the contextual menu of a WebListBox.

The WebListBox is part of a container control that is embedded, at run time, in another container control embedded on the webpage at design time.
The WebListBox has a contextual menu that shows up fine when needed but, most of the time, the WebListBox.ContextualMenuAction isn’t firing.
Is there any know issue with WebListBox and contextual menu?