A few years ago I had a problem with one of my machines and couldn't figure out what was going on so got another machine. Eventually I figured out that the problem was my bobbin itself. I had purchased some of the clear plastic bobbins and did not realize that there were two different ones that looked alike but fit different machines. There is only a very small difference between the two, but with the correct one everything works great while with the wrong one it's a big mess. BTW I no longer use the plastic bobbins because I don't know which ones go to which machine.
Don't know if this will apply to your situation, but it worked for me!