Here are the steps to identify the problem
First, check the outside of the side brush to see if there is any anything entangled
Try rotating the side brush to see if there is any stuttering. (I have attached an instruction video for this)
If yes, you might need to loosen the screws of the side brush to see if there is any abnormality. Please take a picture of it.
If no, it is the problem of carbon deposits in motors. We will arrange a pick-up for the robot at our maintenance point.