I have had the sweet sixteen a couple weeks and can't seem to get the tension right. I am actually in the middle of quilting a quilt, but when I had to put in a new bobbin and then later change the top thread, each time I've had to adjust the tension. It took me a long time to get an acceptable tension on the bottom -- is this normal? I hoped that if I was using the same thread I wouldn't have to keep adjusting. It's very frustrating -- I spend more time quilting samples to get the tension right than I am spending quilting.