When Roomba moves around your home, it sucks up the dirt and debris through the vacuum suction opening. However, in the outer area where your wheel and the brush motor are located, dirt, pet hair, and other things might get stuck. When this accumulation increases over time, it hinders the movement of the parts causing issues with the Brush module.
The Brush Module has a mechanical gearbox inside it, that transfers the power from the motor to the wheel. If there is trash and gunk accumulated inside the same, it will again cause Roomba Brush not spinning issues.