I don't know this particular device, but normally the brushes are in a brass housing and a spring presses them against the commutator, keeping them in contact as they wear down. I suspect the spring is not properly installed or located how it should be. Or if you had the brushes out, could you have put one of them back wrongly so that the curvature of the face of the brush is at right angles to the curvature of the commutator? And while you're in there, make sure the commutator is clean. Some isopropyl alcohol or proprietary switch cleaner on a piece of cloth or tissue usually does the trick.