It seems that in order to nudge, there must be a free space in your yard to move things into. In other words, you can't just swap the positions of things. You cannot move A into B's spot and B into A's spot. You must move A into an empty space and then move B into A's old spot and them move A from the empty space into B's old spot.
At least that is the only way I have been able to get it to work.
So think twice before you fill up that last space in your yard!