Originally Posted by
quiltedsunshine
Metal bobbins go into metal bobbin cases. Plastic bobbins go into plastic bobbin cases -- which includes most drop-in bobbins. Be aware that plastic bobbins get worn-out and get rough around the edges. They need to be replaced regularly.
I agree and think the reason for this is that the metal in plastic may warp the case. I also wonder if there isn't some friction built up with bobbins/cases being different materials that might interfere with the machine computer.