Some rainfall droplets seem not to move

Hi I’m working on a thunderstorm scene for my houses to be rendered in, and, to have rain, I apply a particle system for the droplets (mostly on Blenderguru’s tracks). I have a horizontal plane above my camera,that emits rain droplets (small shadeless white spheres) under Newtonian physics. I render with ‘Vector’ enabled, and in Compositor I apply a motion blur to blur droplets.

The problem: most droplets are blurred, but on every image I have some that don’t move. Any idea to make them blurred?


Seemingly the problem arises at small resolutions, when droplets’ size is only 1 px, increasing the resolution solves the problem.