Geared hub, or direct cdrive?
geared hubs should spin free forward, but not backward. However, a common mistake is to use the screws that came with the disk brake to mount the disk, which will jam the motor up. Many geared motors need shorter disk mounting screws because of their tight internal tolerances.
A direct drive won't spin free. the cogging effect of the coils and magnets will cause it to spin 1-3 revolutions and then stop. when you're actually riding, you should hardly even notice that drag, but it feel supprising the first time you try it.
If it won't make it a whole revolution, then it's probably a shorted coil, or something physically rubbing on the motor.