Jamie Wallen has a good video on
bobbin tension on YouTube and he sells a
tool that can make your bobbin case round again. He calls it a dewarper. The bobbin tension video is a
good starting point and needs to be checked every time you change thread types. The reason being that a thick thread will pull the bobbin case up using the same setting that will let a thin thread leave the case in your hand (that makes more sense when you watch the video.)
I have so far avoided the flattened bobbin case by taking the case out over a rug or an anti-fatigue mat. It's the first bounce that hurts the most.