Have you tried increasing your stitch length? That helps me sometimes with the deco stitches on a quilt. Go up to 3 or even 4. Also push the top layer of fabric back slightly all the time to keep it from stretching so much. I think the poly batting is too thick and stretchy for most deco stitches. This happens to me all the time, because I like to do special stitches on my charity quilts! Remember, the recipient of the quilt will never know or care about the bunching. Good luck!