Sounds like the bobbin thread tension is too tight. Make a little adjustment, one at a time, until you find the right tension on the bobbin case....righty = tight and lefty= loose. If the top thread has little loops showing through the back, the tension may be a little loose for the top thread, too. Try tightening the tension on top a bit and loosening the tension on the bobbin thread, too. There is a nice video on the Internet by Jamie Wallen about thread tension. There is also a good thread therapy DVD by Superior Threads. Sometimes this is offered free for the cost of $3.95 shipping. Anytime you use two different weight threads, you will want to do a test run on scrap material before doing the actual project. Good luck.