Hidden faces and collision issues

I am sure that this topic has been brought up before, but it seems that I can't find it any wheres. I'm creating
a round tower in constructor and placing a spiral stairs around the inside. When I export it out of constructor using the export/torque game engine(map2dif plus) I get missing faces and my collision is not there, or the collision is there but the faces are still missing. Any ideas on how I could fix this problem. Thanks