if you don't have a balanced rythm of machine speed and fabric movement the bottom tension is going to be way off. I've done nearly 1/2 a quilt thinking I really had it going on because the top looked so good! NOT! You just can't move the fabric as fast as you think you should be able to, especially going sideways. Try slowing down but keep your machine going at a good pace, like when you're doing a nice straight seam.