Tammi and Caroline, I've had a 328K since it was NEW, back in 1963. I've never had a bobbin problem with my machine, except for one thing. One of the tiny (VERY TINY) screws in the bobbin case worked its way out and fell on the floor. MAJOR PROBLEM THEN! However, my son and his ingenuity, once he discovered that there was a missing screw, he ran a magnet all around the machine, and before long he picked up the TINY, TINY screw up with the magnet. Bingo, put the screw back in and tried it out -- just a little bit of adjustment, and it ran like a champion. Just a word of caution -- the screw is EXTREMELY TINY, and it was very hard to determine that the screw was missing! It was even harder to find the location where the screw fit. Finding the "spring" on the bobbin case was also quite a trick. Be very careful handling the screw -- it's very easy to lose it and very hard to find it once it's lost! The spring, once it works its way out, can easily drop into the works on the bobbin case, and can jam up the machine. To continue trying to run the machine can cause even more damage to the bobbin case. Because the machine is all metal, it doesn't damage easily!
I've never had a problem with my bobbin case (except for this one instance, and except for a metal bobbin which was damaged when stepped on. Of course, if you step on a plastic bobbin, it will break and won't work at all. I do buy the Singer plastic 66 bobbins and they work very well in my machine. I have no idea why others have problems with the plastic bobbins and I don't! I do know that the Singer plastic bobbins work very well in my machine, but the "cheapo" plastic ones not made by Singer, my son tells me, are absolutely awful! Recommend a thorough check of the bobbin case, clean out and lubricate the bobbin case, and I hope it will be okay!
Give it a try!
Jeanette