Follow the steps listed here: http://www.intelliquilter.com/iQpedia/SuperMotor-Lift-Mechanism-Keeps-Turning.php.
You can check that the screw is at the correct height by looking at the drive wheel. If it is about a credit card height above the track when in manual mode, that is the correct position.
After replacing the screw in the proper position, check your motor belt for tightness. Loose belts are a common cause of bolts becoming un-threaded. If one of the motors is working, you can test by switching the two belts. If the belt is stretched or damaged, replacements can be purchased at: www.mcmaster.com. Use part number 3044K503 and select 5” outer diameter.
Also check the pulleys that the belt rides on for any damage or shininess. If they are damaged, you can get a belt and pulley set at www.quilttech.com.