I bought a Monster MM3 and powered it with an American Ale Works motor. I like it but they do have a little issue with not feeding if the unpowered roller sticks. A quick spin by hand and verification with a handful of grains fixes it but if you forget, picking out 12 pounds of grain kind sucks. I'm buy from them again in a heartbeat, but would buy the MM2.
If it doesn't spin, this tells us the gap, for the malt, is uncorrect. I had to adjust my MM3 for Viking malt, as, apparently, the grains are plumper.
The first gap in the MM3 is fixed, non-adjustable. Sometimes it's best to get the mill spinning before you pour the grain in. I think sometimes all the pressure from the full hopper puts drag on the slave roller. Another idea I had was to extend the hopper funnel on the slave roller side a little bit closer to the center of the gap.
I did just drop some big bucks on the MM3 gear driven unit because shop mills can't be finicky.