I took dragdrop.asm from raedit, commented out things that didn't immediately compile, added a simple window + initialization code and it sort of works. When something is dragged over window, you can see calls to IDropTarget_AddRef and IDropTarget_DragEnter. After that it messes up because most of the code in IDropTarget_DragEnter was commented out. But that was enough for my dumb test.
So, probably you did something wrong with pIDropTarget declaration or implementation.