We've got a "saloon" type dog door, vertical panes that swivel to let our 6-year-old lab in and out. He's never had a problem with this door before but now all of a sudden, he'll go out the door but refuses to come back in. He'll nose at it from outside, and he'll actually come through once in a while, but generally he just sits at the adjacent people door and whines until someone lets him in. This is especially annoying in the middle of the night. Anyone have any ideas?