The way my machine works is if I get threads on the top of what I am sewing I need to loosen my tension as the tension is too tight - and I have had to go all the way down to almost 0 with a heavier thread. Guess what is happening is that the thinner thread in the bobbin is just not strong enough to pull that top thread down far enough (and visa versa). You might want to see if you can find a heavier thread to use in the bobbin. Thread tension problems are frustrating aren't they?