Diagnosis
The drive belt that spins the brush head is likely slipping or has stretched out. It's a common wear item on these mops.
How to Fix It
You'll need to replace the drive belt. 1) Turn the mop over. 2) Look for the circular brush housing in the center of the head. 3) Around the edge of that housing, you'll see a thin, black rubber belt. 4) Gently roll it off the motor pulley and the brush housing pulley. 5) Stretch the new, identical replacement belt onto both pulleys. It's a tight fit, but that's normal.
If That Didn't Work
Make sure hair or string isn't wrapped tightly around the brush axle underneath the pad, creating drag. Lift the brush cover to check.
Heads Up
Don't run the mop on very high-pile rugs or over long cords. That can instantly snag and ruin the belt or stall the motor.