We've seen several problems on older machines where the actuator shaft does not move. The problem is usually one of the following:
(A) If the electric motor feels like it is spinning, the problem is most likely to be the torque limiter gear that is internally worn out. The torque limiter consists of a spring plate that holds ball bearings that slip into detents that are in a plate on the opposite side of the spring plate. Abuse or overuse can wear out the detent plate. The easiest fix is to replace the torque limiter with the latest upgraded version.
(B) If the motor feels like it wants to move but something inside is keeping it from moving, the problem may be the bearing on the actuator shaft. The nut that holds it in place has a tendency to self-tighten which puts too much pressure on the bearing itself and prevents it from moving.
The bearing is in 5 out of the following 6 photos. The nut that self-tightens is the upper piece on the shaft as shown in the top, middle photo. Unscrew the nut a turn or more and the bearing will be free to turn.
(C) If nothing is happening and you are 100% positive that you've double-checked all the electrical connections (check again!), it is possible that one of the motor wires inside the gearcase has disconnected. The solution is to open up the gearcase and resolder the connection or reattach the wire. If you had a 12V battery nearby, you could touch the internal motor wires to confirm the location of the disconnect.
The actuator is a simple device. It is rare that it fails beyond repair. Your first one to disassemble may take 20-30 minutes to get to the state shown in these photos. The second one will take less than 10 minutes to fully disassemble.