DIAGNOSIS The green end cap on one side of the rubber brush is either worn, cracked, or not fully snapped into its slot in the robot's chassis.
FIX Remove the brush. Inspect the green end caps on both ends. If cracked or deformed, order new ones (they're a common spare part). When reinstalling, you must hear and feel a distinct *click* as the cap snaps into the metal bracket inside the robot.
IF THAT DIDN'T WORK The brush itself might be the wrong model or warped. Ensure you have the genuine iRobot AeroForce Dual Rubber brush, not a third-party version. A bent brush will not stay seated.
HEADS UP When removing hair from the brush, pull it off from the green end cap side, not by yanking it through the brush's fins. Yanking can deform the cap.