Exact size hole not allowing an object through

In my project I have a spot with a square elevator platform and a square shaft of the exact same size. At the bottom of the shaft is an entrance for the player. As it goes up the elevator gets stuck on the corner of the ceiling on the entrance and refuses to go any further, though if I move the corner away from the shaft by the tiniest amount it works again. Anyone know why this is happening and how to fix it?