In addition to what the others have already said, check your bobbin casing one more time with a strong flashlight for a teeny, tiny piece of thread stuck somewhere. Not mechanically knowledgeable enough to know why, but a friend had this issue on her older Kenmore. It took me over an hour to find the little bugger. No amount of tension adjusting would fix the problem. It was that stupid little piece of thread stuck. Get out a pair of tweezers and pully that puppy outta there. Good luck!