One thing I haven't seen mentioned here is ironing your quilt sandwich after spray basting it together. It sets the spray and also shows up any excess fabric that perhaps wasn't smoothed out in the basting process when you layered it. Even after ironing, it is repositionable, and you can get it nice and smooth. As others have mentioned, you don't want to stretch your backing or top while sandwiching, because when you remove the tape, if that is what you are doing, the layer you have stretched will just snap back to it normal size and cause puckering fabric in the layers. I have used both Warm and Natural Cotton and Hobbs 80/20 without and issue. I have had issues with polyester or wool batts as they have shifting within the batting itself, so stick with the cottons or almost all cotton like the 80/20. After I sandwich the quilt, I always ditch the quilt on my domestic (9" harp too) with my walking foot, starting with the outside border, working in or the longest continuous lines. If have done several King sized quilts this way, without a pucker. Once the quilt is stabilized with your ditching you can go anywhere on the quilt without an issue. If I do an all over design without ditching, you will need additional stability as mentioned with pins or water soluble thread on a big quilt. And keep your quilt supported when working on it. Anyway, a bit of repeated advice here, but keep trying, it's doable. Good luck!