Colliders won't work

Hey everyone,
I have a problem, i’m making a racing game for android devices, and for car i make box collider to not fall trough track, with box collider on, no falling trough, but when i put wheel colliders on wheels, car falls trough track, why? In Unity Pro 4.1 version everything was ok with wheel colliders, but in Unity Pro 4.3.1 wheel colliders won’t work for me, Please help!

I still need help with this…