Originally Posted by
NJ Quilter
It sounds like you could have a piece of thread stuck somewhere in the upper threading path. Since it's not visible I would guess the tension discs. You can try using a piece of dental floss to clean out the path on both/all sides of the discs and see if that helps.
I have had this situation happen on both a top and bottom bobbin machine. It can be a nightmare because sometimes it is not visible. I have even had to use a pin to get a teeny piece of thread out of the tension springs in the bobbin.
Good luck. One time I never did find the piece and had to take it in for service. I'm sure they just took the bobbin case apart and removed the offending thread. But I was getting nowhere except more frustrated.