Like others have said it could be caused by several reasons and combinations of variables. I once had a quilt to do that had mainly batiks and I was having some problems with stitch quality, I troubleshooted everything I could think of. Finally changed the needle from one of the expensive titanium needles to a regular one and the stitching looked great. Just want to mention this if others have the problem. I actually think the titanium finish did not allow for a good contact with the fabric. Batiks are tricky and finiky (sp).