AFAIK, all stir bars have a magnet embedded inside. They should be stored in pairs stuck together to help preserve their magnetic properties. I have yet to see a stir bar that was not magnetic, but from what I've heard, they can lose or weaken their magnetic properties if subjected to strong shock such as if dropped repeatedly. Thrown stir bars are usually an indication of poorly aligned or spaced drive magnets. Stir bars can be damaged though, so that is a possibility, but I doubt that the problem is due to them suddenly becoming magnetized when before they were not.