I've had that happen. I agree, something with the top
tension. Couple years ago had trouble with my machine and
took it to my dealer about 35 mile N. of me. After checking gobs of stuff he took the whole cover off of the
machine and over on the right side (under the 'wheel') there
was a birdnest of thread that would fill the palm of your
hand..I mean so big it was larger than a coffee cup. Neither of us ever figured out where that thread came from
or how it got in there. Puzzleing to this day.