Diagnosis
The side brush motor is obstructed or has failed. Hair and string commonly wrap around the brush's central post, seizing the mechanism.
How to Fix It
Remove the side brush by pulling it straight off its post. Inspect the post and the brush's hub for wound debris. Clean thoroughly. Manually spin the brush on the post to ensure free movement. Reinstall.
If That Didn't Work
The motor gearbox under the side brush cover may be jammed. Remove the two screws securing the side brush cover and inspect for a debris jam in the gears.
Heads Up
Check and clean the side brush post weekly. A seized motor will not be covered under warranty.