Strange Collision Bug.

I have no problem with visible mesh collision on a complex shape. Like, I can walk around inside building mesh which is like 10.000 polygons with collision set to visible mesh. However, If I put several simple box meshes on the ground to walk over them, since each boxes touch each other, I fall through some of them. I created collision models for the box, named it as collision, still the same problem.

So we can't have proper collision on copy/pasted meshes that are aligned to each other?