what size needle are you using? I ask because recently while Fmq on a quilt I began having stitch formation issues that couldn't be resolved by any of my usual means. I had decided to try out a new kind of batting and was also using a batik for the backing fabric. What finally solved the problem for me was going up to a size 100/18needle. I usually use a 90/14 top stitch or the 90/14 hlx needles in my Pq1500s, don't know if it was the new kind of batting, or the batik backing, or the combo, but the larger needle solved the problem.
Rob