[BBray_T1296]

In some motors like those and the Fisher Price motors, if the mounting screws are too deep they will jam into the internal coils and mechanically halt the motor. Otherwise, it is possible that some of the little fan blades at the back have broken off and are in the way. I cannot think of any reason a healthy motor would fail other than these reasons (the motor will run just fine with a few fan teeth missing, so long as they are cleared)