Liberation. Test that each of the little graphite looking chunks (this is a carbon brush) slides smoothly in it's little spring loaded track with a hand tool or a ballpoint pen. Resistance means you have located the source of your headache and can show your partner how clever you are. If both slide smoothly, I'd recommend you keep following the tutorial because you're already here and it might become an issue later. (Note that the bottom brush in this image is stuck in a retracted position.)