There are two types of plastic bobbins: Good ones, and junk. If you buy them at JoAnn fabrics or Hancock or elsewhere it's hit or miss what you'll get.
I bought a batch of plastic Class 66 bobbins from Sew-Classic for those machines with plastic bobbin cases and they work great. They also fit and function just fine in my older to antique 66s, 99s, and other machines.
It just all depends on the quality of the bobbin.
Joe