adding to what Feline has said, i once had a bobbin that I wound way too tight--still not sure what happened, but the thread would not pull off consistently. Do you use a Towa gauge? it will help a lot with checking that each bobbin is consistently wound and pulling off without problems. also, besides the bobbin spring(which doesn't look at all like a typical spring so don't let that confuse you) you might check that your thread is setting under the tension arm of the bobbin ok--it's possible that it's worn out. I do sometime use the magic genie "washers"--you don't use a bobbin spring with them, but i truly believe the bobbin spring works better. good luck--machine problems are SO annoying!