Nikon F3HP, Nikon MD-4 Motor Drive. I checked it before loading and shooting a roll of Tri-X. I had the orange dot set on the MD-4. Set the two switches, thought I rewound, and ended up exposing the film- it had not rewound. I did open it up without film in, released switch 1, slid switch 2 up- I can see the coupling spin. But for some reason, it is not engaging the film cannister. I can rewind manually, after releasing switch 1, but I'd love to automatically rewind. Any suggestions?