I seem to rather suddenly be having an issue where my followers don't want to follow me through certain doors, like leaving Whiterun. I go through the Whiterun gates and my followers stay behind unless I 'wait' for an hour for them to catch up. Inigo (a modded follower) has done this leaving a dungeon, too.

I've never had issues like this before ... what could cause it? And, is there a fix?