Manufacturers strongly suggest to 'activate/magnetize your bobbin case' and never use plastic bobbins. Use metal bobbins only. If this is not done there will be bobbin issues eventually. My sewing machine repair person taught me this. Of course he may have cheated himself out of repair charges. One of my manuals states this as well. I learned this when plastic bobbins first came out and we were deciding which to use (metal or plastic). I know a metallurgist (metal engineer) and he said it makes perfect sense.