Mine was doing that same thing after about 3 batches. I pulled it apart, cleaned the bearings up on the rollers, and once back together, good as new.
Did you take it completely apart? How did you get the knurled knobs off? They must unscrew.
The phillips head type screws along the back plate basically lock in your mill gap knobs (which are basically eccentric cams).
You need to unscrew all the screws on both faceplates (those would be the thinner sheets of aluminum), i.e., front and back sides.
You need to unscrew the two larger hex screws on the undersurface of the particle board base.
Then you should be able to disassemble the box-like mill housing.
Then do as weaze suggests and clean the axle and sides of the rollers.
You can get cheap o-ring rubber gaskets to put on the rollers so that the free roller will be driven by the drive roller, the problem is that the rubber tends to get cut up and eventually breaks off and ends up in the mash. (doesn't seem to cause any untoward efects though). It gets dumped out with the spent mash.