I'm quite new with SkechyPhysics, so I could be wrong, but in your model there are at least to problems:
1- nut is not centered referring to the screwing axis (it can be clearly seen when you click on play)
2- bold has not a real thread ( ///// ) but a sequence of pararllel discs ( |||||| )

Try to fix these two bugs and maybe you will solve it at all