I've seen this in a few games so the fact that nobody has made a workaround is kinda sad... anyways the problem is that big and sometimes (but rarely) small units will not obey a move order even though there is infact room. To prove that there is room for it the unit can move inbetween the location if you tell them to move past it, and then press the stop button to make them stay in the tight spot.

Another thing with having units tightly packed together is that it seems I can't load my units into a landraider... which is very annoying