Sometimes quilts just roll up onto the pickup bar unevenly. Sometimes it's the quilt top which is pieced imperfectly. Get in the habit of checking that with each advancement. I stand at one end of the frame and eyeball down the quilt, looking to see if the seams are straight all across. If not, you can "roll" the quilt up tighter where needed along the takeup roller with your hands. You can actually do the opposite, too, by "loosening" the quilt in spots with your hands, working the rolled up quilt on the pickup roller. Hope my description makes sense.