The two most common reasons for this problem are slow machine and fast hands. If you speed up the machine and make slow even movements, you should be able to avoid this problem. By swinging quickly in an arch motion, you are moving faster at that point than the bobin can catch up. It really takes concentration on even speed to get your back to look as good as the front. Catch yourself from moving quickly when doing a circular motion. This seems to be the most comfortable movement and we tend to quicken without realizing it. Like any other skill it takes practice and alot of unsewing. Ask me, I know this all too well.
Good luck and practice practice and then practice some more. You will be pleased with the result. :wink: