First the backing must be square before loading it to a frame. And make sure it is pinned with the center of the top of the backing and the center of the bottom exactly opposite each other. Each time you advance the quilt sandwich look underneath to make sure no puckers have been rolled in the backing. If you see a pucker, loosen the tension, pull on the backing until the pucker comes out, tighten the tension, and proceed to quilt.