I haven’t seen a post on this one and thought of posting it for fresh users so the majority of you guru’s should please not be offended with this modest post
You are trying to move a door from one wall to another by selecting and dragging it with the mouse – and it doesn’t work.
The right way to do this is to select the door and click Move from the tool bars - you will get an option Disjoin on the options bar which after you select it, will let you move the door to another wall. Same applies for the copy command.
well, it is a simple tip but might save some research time..