David,
First, you need to determine if the step motor is activating, and it is not able to retract the step because the step linkage is binding. The step linkage can bind from rust, corrosion, or misalignment. Misalignment is usually caused by bent linkage. If the motor is not activating, you need to determine if it is being supplied 12V power at the proper time. If the motor has 12V and it does not run with the linkage disconnected, you need a new motor. However if the motor is not receiving 12V, you need to troubleshoot the control circuit.
Gerald