Recently the somewhat crotchety man who cleans and adjusts my sewing machines gave me a big frown and said, "You're not even using the correct bobbin size." (Bobbin size?) I always thought as long as the bobbin is round and has a hole in the middle and doesn't spit itself out of the machine it was okay to use it.
What kinds of problems can be experienced if you're using the wrong size? And how do you know what size your machine uses? (My manuel doesn't tell me.)