If you have turned the tension all the way up and still have the problem, it sounds like you need to very carefully raise the tension on your bobbin case. That is the last step to try and takes a very tiny turn of the screw. I have only had to do that a couple of times in many years of sewing. Be very careful, as the screw is very small and easy to loose. Having loose bobbin threads is a very frustrating problem. Good luck.
Sue