Particle emitter problem

I’ve made a cat litter box for a phone app I’m developing and I’m using a particle system for the litter particulates. But even though I’ve defined collision on both the sides and the bottom of the litter box, some particles are still shining through, it seems.

Any tips on how to avoid this?

Blender 2.65a (64-bit) on Windows 7 64-bit
Rendered in Cycles: 50 samples

Wrong place for this thread. You need to post this in the Particles and Simulation thread.

Thread moved to Particles and Physics Simulation Support

Solved it.

The plane of the bottom part of the litter box extended a little bit too much to the sides, probably because of the Solidify modifier used on the bottom part.

I cleaned up the model with a subsurf modifier on the bottom and smooth shading for both the top and bottom parts.