In my 2d top down game cube colliders are only colliding on 2 opposite sides. heres a pic to better explain it.
The cube has a box collider and so does my player. My player also has a rigidbody.a
I hope I have supplied enough information for a decent answer, please ask for more info.
Make sure your colliders match according to the table in the bottom of this page:
Not sure if this is the case, but you can run the game, pause and then switch to the scene view. Check how the colliders look like while running the game. It happened with me one time that the colliders where somehow displaced when the game was running and they looked just fine when I was editing the scene without the scene running... Besides that it´s really weird!