When you are working on tensions it takes patience. Generally I take it apart, clean it and re-assemble it. I sit there with the page from the book - I enlarged it. Carefully follow the instructions. Then do the same with the bobbin shuttle. Adjust it like Muv says. It may need some clean up, too. After all that, it may or may not be the tension. Some times a bit of dried up oil in the tension or a thread race can make things not run smooth. Some times a burr somewhere - hook or the needle hole. A bad needle or the wrong needle or the needle in wrong for the machine can mess things up. A bobbin wound wrong or a bobbin out of round. So many things. You may need to pm with Joe some time as you work on it.