When motor is not turning this is mostly due worn out drag/brush contacts. On the edge of the commutator sections contamination builds up resulting from the spark when brush contacts jump from one part to another. On which side the edge get contaminated depends on direction of rotation. Nevertheless brushes are often so far damaged, reversing direction works only (if at all) for a very limited time of operation. By default motor is installed with rotation resulting in pulling brushes. If rotation is reversed by swapping polarity, brushes get pushed and since they are most likely in bad shape already, chances are real brushes get crumpled.
Better recommendation for long term solution is to replace motor by a new Mabuchi RF-500TB-12560, they are readily available on internet. When connecting, pay attention to direction of rotation making sure a pulling force applies on the brush contacts for longevity.