Trouble with drag-n-drop technique

Hi developers,
I am developing a host subsystem for an embedded plugin with Qt. All is okay, but I have a problem when implementing drag-and-drop with plugin view. I have a “frame” inheriting from the QDialog and the IFrame (twice inheritance), and an “interior” created by the plugin as IView. The interior successfully attached to the frame, result is Steinberg::kResultTrue. However, drag and drop mouse events from application successfully reach the frame, but do not reach the interior. What am I doing wrong? Is there a way to “forward” these events to the interior?
Any ideas are much appreciated.
System: Windows x64, Qt 5.11, MS Visual Studio + Qt Tools, VST3 SDK 3.7