Take off the upholstery and examine the rollers, the track, and the gearing. You'll also see that there are adjustments that can be made in the mounting of the operating mechanism to the door. In addition to the built-in adjustments possible, washers can be used as shims under the mounting points.
Be especially careful in handling the operating mechanism, as they are very scarce and very expensive to replace. Repairs are possible if you are good with metal. Some guys have remade the entire gear plate with good results.