It took me quite a lot of snarled and broken thread to figure out that to correctly mount the bobbin in my "new to me" Bernina, I needed to open the little latch. Not sure how I got it to work correctly the first two or three times I changed the bobbin.
I later had a moment of heart failure when it would not advance the fabric at all - checked that I didn't have it set to wind bobbins, moved the feed dogs up and down, dismantled and re-cleaned... then remembered that I had set the stitch length to 0 in order to end a seam. DUH. Runs great now!
Cheers, K