Make sure your brass is decrimped before you use the uniformer. Even if a little crimp is left over the tool will grab, and the result is marred side walls in the primer pocket, which in turn could cause primer gas leakage.
One of the weaknesses of any uniformer used on the Trim Mate or even by hand is, a good job is only achieved if you are careful to place the brass straight in vertically...anything else will make the tool grab the brass and bang up the walls. I think this is the reason RCBS chose to use a low speed/torque motor in the Trim Mate to lessen the damage to the pockets.
If you do experience the "case grab" let go of the case and turn off the machine....carefully remove the case, examine it for leftover crimp, use a reamer if need be, and try it again.
RCBS's wire pocket cleaner is also designed to go straight in. It's effective, but don't angle it and scratch the side walls. Minor scratches don't seem to hurt anything, but if you are a "bull in the china closet" type of guy, use some restraint.