I had a bit of trouble with a Japanese 15 last night. The bobbin case and bobbin would roll out just as soon as I put it in. Some times it would actually stay in. I was going CRAZY... Things can cause - maybe more - maybe someone else will know...
ONE. The take up lever has to be ALL the way to the top of the space (this is true on my modern industrial sewing machine, too - it has a mark on the machine for where the bobbin will go in and it is the same position)
TWO: Be sure the bobbin is not bent or warped and fits in the case flat - any defect can cause the whole thing to roll out.
THREE: Be sure the thread is clear of the area - it should be sticking out - not in the path of the bobbin.
FOUR: Make sure the tail from the bobbin winding is cut off and not making the case stick out too far.
I'm sure there can be other causes of bobbin case and bobbins rolling out of a machine.