Did you cut the com?? if not that could be your problem. Check that the brushes snap back on the com when you pull the brushes back a little, ( with brush springs left on) Checking for binding in the brush hoods.
Are your bearings good? These motors are good but don't over gear it. Always start lower in your pinions than work up on spur than pionions. I always go one to two teeth lower than the manual just to be sure. I can get over 40 runs on xxx silver brushes. Good luck. Mark.