I have a friend who had an older machine that her husband bought her years ago. When it wouldn't work, she asked a woman at a repair shop what it would cost to get it fixed. From what she told the woman, she was told that it was probably the timing that was off and would have to be readjusted and would cost $100.00. Since her machine was quite a few years old, she thought it would be better to get another machine. The new machine worked OK but she could not get the tension right. She asked me if I'd come over and see if I could fix her machine. I asked her if she ever cleaned her machines and oiled them, and she said no! I took the tools I needed and the oil with me, and started on the older machine. I cleaned it out, and it had a lot of felted dust and lint in the bobbin case. I took the bobbin case out and cleaned under and around it as good as I could, and under the feed dogs. Then I oiled the machine after I got it back together. She was so happy when I was able to get her older machine working. And all it needed was cleaning and oiling. I did the same to the other one, and also worked on the tension and got it to working OK too.
I was told later that pipe cleaners work well to get down around the parts under the bobbin case, but I haven't tried them yet.