Why does the bobbin case pop out & form a nest underneath my sewing machine?
All of the answers have good points, but honestly, you've probably got the bobbin in wrong. No one ever seems to know this... a bobbin when it's pulled out of the case needs to pull against the open edge and not with it... it's hard to explain, but it needs to unwind in a specific way - either clockwise or counter-clockwise.... each machine is different, but depending on the bobbin case that you put it into, if it doesn't work one way, turn the bobbin over and let it come out the other way... usually fixes this like a charm! Yes, your tensions could use some adjusting too - bobbin tension is hard to adjust and it a trial and error by screwing down that little screw found on the case. I doubt it's the bobbin tension, so try what I suggested above. If it works, let me know!