It could be that your bobbin tension is too tight. Try loosening the screw on the bobbin case a tiny bit (if it were a clock face, only about 5 minutes at a time) to see if that helps. Although this video is produced by Handi-Quilter (mid arm machines) it show what your tension should look like: http://www.youtube.com/watch?v=KrGVgMRBDyA

There are lots of YouTube videos explaining bobbin tension.

Also, if you are free-motion quilting and getting bad tension on the curves, try slowing down your stitching.

I am trying to quilt a baby quilt. The top looks good but when I flip to the back, the top thread is pulling through to the bottom especially on curves. I have tightened the tension from 4 to 4.4, made smaller stitches, and still get the same results. HELP!!