If you disconnect the roof from the cable and move it around by hand, does it slide easily? That should give an indication of whether it's friction in the roof (i.e. the angles and fuzzy nylon sliders) or with the motor. Did you grease the lifting angles and the sliders, or just the stainless steel rails? Does the cable pull out and go back in easily? If all those are smooth and well lubed, I would look at the motor.