Suggestions:
1. Check or clean commutator. There are some good postings in this Forum with references.
2. Measure voltages at the motor terminals. They are supposed to correspond to the motor rated voltages.
3. Check motor winding coils resistance/impedance for continuity.
4. Check brushes, braces, etc. for their correct positions and quality.
5. Make sure that the shaft load is as designed, and that there is not any overload.
This may be just a start with more to follow.