DIAGNOSIS The debris extractors (the comb-like fins next to the brush roll) are jammed, or hair is wound tightly around the brush roll ends outside the cleaning path.
FIX Flip the robot and open the brush roll cover. Remove the brush roll. Look at the two gray plastic extractors on either side of the cavity. Manually spin them—they should move freely. Clear any hair or thread wrapped around their axles. Then, inspect the very ends of the brush roll itself, under the plastic caps, for hidden hair wraps.
IF THAT DIDN'T WORK The brush roll motor or its sensor could be failing. Listen closely when you start a job—if you hear a faint whirring but the brush doesn't spin, the motor isn't engaging. That's a hardware fault requiring support.
HEADS UP Cutting hair off the brush roll with scissors is fine, but never cut debris off the extractors or you might nick them. Use the included cleaning tool to hook and pull threads out.