I was dealing with this exact same problem just yesterday. Using a walking foot; fabric/quilt supported on big table; stitch lengthened, etc. Pinned in a gazillion places... still puckers! After picking out waaay too many stitches, I gave up and went back to the regular presser foot and guess what? It looks much better!
I'd even considered just tying it off. Tried that and didn't like the look.
Honestly think each and every quilt calls for something just a 'touch' different than the last.
Please let us know what ends up working for you.