The brushes may be worn down too far and no longer make contract with the commutator. Inspect the brushes to make sure that they are still making contact with the commutator. Refer to manufacturer s recommended brush length chart. Controller may be defective. Verify voltage is coming out of the controller.
Motor runs but loses power.
Verify the load has not changed. Measure the amp draw of motor against the full load amp rating of the motor. If the amp draw is higher then rating, motor is undersized for application. Motor controller not properly set. Check controller manual for adjustments.