I had this problem with the little I Spy I just quilted. I basted it with Elmer's washable and obviously didn't get the top "tight" enough. My batting was not too fluffy; in fact, it is, I think, a little too thin (a single layer of polyester). Where I noticed it most was the sashing, which was cut across the fabric (strips cut width of fabric or from selvedge to selvedge). I also used a walking foot, and the serpentine stitch on my Brother 450Q. I've used that stitch for many quilts and never had this problem before. The sashing was Kona white, too, so not a cheap fabric.