Brushed DC motors are widely used in many applications, including the servo motors on Nybble. They are simple in structure but also have weak points that would fail most frequently.
The brushes are designed to push against the rotor to form a closed circuit. So friction is inevitable. The material will wear out after certain period of use. Once the brushes are worn out, it would either lose contact with the rotor or stuck in the rotor's contact plates. The broken pieces may even fall in between the rotor and the wall to stuck the rotation.
There's a quick fix to bring a dead brushed DC motor back to work. Basically you open the motor and inspect the situation of the brush. Usually the brush has a longer base than needed. So it's possible to push that part out and reform good contact between the brush and the rotor. Considering the probability of failure for the two brushes A and B. After we fix brush A, B is more likely to fail before A fails again. So the lifespan of the motor can be doubled, without buying extra replacement parts.