Collision problem

Hey there.
I have a problem. I use blender with Nif Script. I want to make Fallout New vegas models more realistic by editing their collisions. I started with a bus station, however, everytime I export my moel as a .nif file, only the roof has collisions but not the sides.

Here’s a picture (green = collision):

What’s wrong? The sides have the same settings as the roof…

ALSO: I read here that I can combine different collision parts. Where is that option?