I'm on 120+ batches on mine and it has recently first started to show signs of slippage. I find that using my drill on high speed is most likely the culprit. If I start the crush with a short slow pulse after I put a couple punds of grain in (but not the whole grain bill), the rollers "grab" the grain, then I stop and fill the hopper, then I go at a slower speed.
I'm thinking the high speed "glazes" the grain and rollers.
Will try 120+ more batches with my new technique.