'Lectric motors don't run with the kind of fine tolerance that IC engines do. If it's really mechanical and not magnetic then the motor is defective and should be returned.
Some of the motors I've handled have had pretty strong magnetic detents, though. If you've only handled something like eFlight motors before then one of the really stiff ones might seem broken. I'm a determined motor-fondler, and I fly with a bunch of RC guys who show up with all sorts of different stuff at the field, so I know whereof I speak.
It's hard to describe (assuming you're not sure), but if it's mechanical it should have more of a "grinding" quality to it, and it won't necessarily line up with the magnets.