These carbon brushes are a common replaceable item that wears out with use over time. As good practice always carry a spare set of carbon brushes to ensure you have a set handy to replace worn out ones when the machine starts to emit sparks or when you notice the performance deteriorating.
Note there usually two types of carbon brushes to choose from: With automatic stop and without automatic stop. Carbon brushes with automatic stop (Sensei Plus) have a spring through the length of the carbon brush. When the carbon brush wears out, the spring gets released, the current to the carbon brush gets interrupted and your power tools switches off at once. The advantage: No additional wear and tear to your power tool. Carbon brushes without automatic stop (Sensei) do not stop functioning at once, but performance of your power tool will deteriorate rapidly.