in every bobbin case there are two screws. one holds the tension and that one is teeney weeney. the tension-holder itself is fairly wide, almost as wide as the case, and has the slot that the thread comes out of. that is the tension spring.
the other screw is slightly larger. it holds the the tension spring itself to the bobbin case. when you adjust the tension, you always use the teeny weeny screw. try never to use the larger screw. you should almost never have to.
i tried to keep this clear. hope it worked. now -which screw do you think is missing? is the case and spring falling away from each other? is the tension just too loose? that will tell you which one is missing.
i really hope i helped you.