Made sense to me until someone much more knowledgeable than I pointed out that spinning the arm by hand is not how the motor works when the arm is energized.
I may be repeating this statement from some time before;
Spinning an arm in the set-up, by hand, without the brushes, is important.
If you know it's a setup with a north and south magnet, the bearings aligned, the arm has side play, the stack is not hitting the magnets, and the comm is not hitting the brush hoods, but the arm doesn't spin freely (like it's hitting the brakes), most likely there is copper chips, or other conductive material in at least one comm slot, shorting out the arm.