One thing that was discussed here a while back is the fact that there are 2 different bobbin cases for the 301 / FW. It looks like one superceded the other (45750 and 45751). I suspect that what happened is that the replacement case was modeled on one or the other case, and the machines that have trouble are the machines that prefer the other case, but I've never been able to confirm because all of the FWs I've had here have always had the older of the 2 part numbers.
ETA: Some portion of the trouble can also be user error. Failure to properly adjust the tension - and I've had bobbin cases come from the supplier adjusted wrong - will look like a problem bobbin case, especially for someone who's never adjusted bobbin tension.