Check controller manual for adjustments. Motor may have an open connection. Inspect the armature for an open connection. Brushes may not be seated properly or worn beyond their useful length. Verify that the brushes are properly seated and measure their length against the recommended brush length chart.
Motor takes too long to accelerate.
The accel trim pot of the controller should be adjusted. Brushes are worn. Verify brush length. Bearings may be defective. Inspect bearings for proper service.