I commonly find shuttle / bobbin trouble with thread breakage on a Vibrating shuttle machines. I'll wind a bobbin, place in shuttle and strip off the thread by hand. Oftentimes I find the thread catches in the shuttle which would break the thread if in the machine. This could be due to uneven bobbin winding, shuttle problem, or the lint you pulled out. Upper tension problem are much easier to spot since not too much can go wrong.
Once the shuttle gets a notch or knife edge at the edge that wears on the race
It's best to replace if possible.
Jon
Last edited by jlhmnj; 12-16-2013 at 09:52 AM.