Bobbin Case will not snap into housing

Question

When I'm sewing and the top thread wraps aroung the bobbin case it catches on the arm of the bobbin case. The arm swings out and the thread keeps wrapping. When I put the bobbin into the machine the arm of the bobbin will not "snap" closed. Thus, it's arm is not tight up against the machine. I purchased a new bobbin case, but it doesn't work either.