[wireties]

We are doing much the same thing but the rollers (usually) turn. Look at the rollers first. How difficult is it to rotate the rollers (with polycord-induced tension but w/o motors attached)? If you can't easily turn them with your hands (but they need not spin freely), the polycord will slip. Try grease, better alignment, change polycord from 10% to 5% - something will work because ours work just fine.

Good luck!