Does the car run on inspection? If it does you need to tach the car and make sure that the speed of the car matches the command velocity. I've seen this fail if the brake is dragging or even if the machine is empty of oil causing excessive resistance to the motor.
Is this an AC or DC job? If you have not already, check the encoder power supply. It may be 12 volts or 5 volts depending on the equipment you have.
