won't open

I have over 10 years old T-3 pool cover. It closes with no problem but won't open. All clutches are engaged properly. The rod that's going into the main drum is spinning. The other end of main drum does not. While motor is running to open the cover I was pushing the drum/vinyl by hand to help...

I'm looking for suggestions about likely causes for this. The cover engages to close (though it's already closed - hmm) but there is no sound at all when I try to open. The key-switch is fine - I confirmed continuity down both legs - and obviously the system has power since it will try to close...