Do you mean the bottom bobbin thread is too tight causing the bottom stitching to kind of gather?If that is the case, is there a tiny screw on your bobbin case? If there is a screw, it is tighty rightly and lefty loosey for the screw on MY bobbin case.Before touching the screw........note the position so you can put it back to the factory setting when done quilting. Make up a few sample sandwiches and turn the screw a litte for each sample sandwich until you get a nice stitch.
If you are talking about puckers in the back as you quilt because the back fabric is moving, that is why I use Hobbs 80/20 fusible batt for my quilts. I don't get puckers with the fusible batt. I like the 505 basting spray also for the same reason, no puckers on the back.