too many particles trapped on wall with tracking
 

I am currently doing particle tracking in a gas solid flow in a geometry with swirl (something similar to a cyclone). I have put the wall boundary conditions to "trap", and am not using interaction with the flow field. About 50% of the particles are trapped on the wall if I make them small. However, even if I make them tiny (1e-9 m) and they get the gas density, still 50-60% is trapped on the wall, which should be 0% since they become tracers. I guess this has to do with the mesh size near the wall (polyhedral), but find the magnitude very large. Does anyone have ideas how to solve this?