Fall through interiors?

This is very frusterating for when I'm testing. When I add a new DIF to my game, and I go over to it, I just fall into it. Usually only the one part of it has collision, and I fall through the rest! It doesn't do this in any other TGE(A) game I've used, so whats wrong? When I add to it, the old part gets collision, and the new part has no collision! Why does it do this, and how can I fix this?