This is so because the DragOver event is handled inside of the ScheduleView. In order to execute your method even the event is handled internally you will need to use the other overload of the same method:
DragDropManager.AddDragOverHandler(ScheduleView, OnDragOver, true);
True indicates that the handler will be invoked even if the event has been handled. However I would like to suggest you not to handle the event inside of your handler, as this may cause the ScheduleView to misbehave.
Hope this helps.
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