I had the same problem with loops of thread on the back of my project and it was so frustrating. I decided I was going to search online until I found an answer. The answer for my problem was - the presser foot must be down in order for the upper tension to work and prevent the much hated loops. Even though I wasn't using a presser foot, I still had to lower the bar thingy where the presser foot would be. Not very good with technical terms am I? This solved my problem - hope it helps yours.