It is most likely your tension. Signature thread is great, probably not that. When I first started after a few quilts I put a large muslin sandwich on the LA and worked with different threads and tensions. I did this for two whole days, but when I finished I had a much better understanding of the thread tension relationship. If you do not have a Towa (sp) tension tester consider buying one. They make finding the correct tension soooo much easier.