Check this.

Make sure you put the black cup back on the end of the motor shaft and that it is pressed all the way on. Make sure the RPM sensor is plugged in. If neither of these are the issue, then you probably did not have the motor shaft pushed all the way back when you tightened the coupler after replacing the worm gear. Loosen that coupler up and from the front of the motor (opposite end in which the coupler is on) push the shaft back as far as it will go. Hold it there while you tighten the coupler back up and then try it.

Does the light blink a few times when your press the button and it moves @ 6 inches? My problem was I accidently pulled out one of the wires from one of the sensors located at either end of the garage door frame at the bottom. Mine is working fine now. I am in search a manual tho. Mine is pretty much destroyed and just wanted to keep one handy. Good luck hope this helped.