Bobbin gets tangled up
Try taking the covers off again. Now check under and around the bobin case, thats the black plastic bit that holds the bobin. Look for any fuzz or stray bits of thread inside the machine and and around the feed dogs and under the needle too. I use compressed canned air to get in and really blow everything clean. For some reason if the machine is not totally clean, inside the bobin gets tangled up. If you have had even one knot, odds are parts of it are still floating around inside there somewhere and stopping some little lever from moving and jamming your whole machine. Clean under the bobin case too since lint can left it up and keep the bobin from being perfectly level. Also inspect your bobbin case to make sure it isn't scratched along the edge that rotates back and forth. Even a a tiny scratch will cause it jump its tracks and your bobin will lift up and become jamed. In that case you just have to buy a new bobin case. But most of the time your culprit will be some stray bits of thread or lint.
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