Particle Collision via traces not working.

I’m trying to get a nice, working snow particle, and I’ve decided to use the particles given out in the winter mapping contest. However, my map is not 100% outdoor and I can’t afford to just place particle emitters on the map without having clipping with snow and fog and such. Because of that, I’ve edited the particles myself, and with the information provided here:

I believe myself to have done it correctly. However, the snow particles themselves don’t seem to collide with anything! (Tried both INTERACTIVE and DEBRIS) The fog effects collide fine and delete themselves, it just seems to be the snow.

Does anyone know what’s up?