DIAGNOSIS: The brush isn't pushed on far enough, or the locking mechanism on the brush shaft is worn or broken.
FIX: Remove the side brush. Look at the robot's side brush motor shaft—it has a flat side. Align the flat side of the brush's hole with the flat side of the shaft. Push it on with firm, direct pressure until you hear and feel a definitive click. Don't just press lightly; it needs to snap into place.
IF THAT DIDN'T WORK: The plastic locking tabs on the brush itself are likely broken. Hold the old brush and try to wiggle the center piece; if it's loose, the brush is trash. Order a genuine Ecovacs replacement side brush—the fit and plastic quality of third-party ones are often poor and cause this exact issue.
HEADS UP: Regularly check and clear hair wrapped around the side brush shaft. Long hair can build up under the brush, preventing it from clicking on fully and eventually damaging the motor.