I found that when I spun my brush, there were sections where it was hard to turn. I think that if the motor tries to start in those positions it would get stuck and the vacuum would "pretend" to clean.
I took the motor and belt assembly out (requires removing the vacuum motor and screw on the bracket of the right section camera mount.) There's a single screw holding the motor and belt assembly and you can slide it out to the side of the robot. The belt was loose so it was cogging on the pinion every few revolutions. I opened the clear cover and reseated the screws that mount the motor. This gave tension back to the belt and the brush spins smoothly now.
Good luck to you if you try this fix! The robot has many wire harnesses, be careful not to pull or pinch them!