By using metal bobbins only in the beginning (when machine is new) and doing this (activating/magnetizing) 'sets your bobbin'. You can’t use tension control to keep your bobbin operating correctly in the casing. Tension control is for thread only. When plastic bobbins first came out they had a small notice on the packaging that stated ‘not to replace metal bobbins’. Plastic bobbins are ‘temp bobbins’. They are for taking with you if you need to take your bobbins somewhere or to loan to someone etc. The plastic bobbins were very cheap and not a great loss during a time when metal bobbin prices increased a lot. I saw it happen, metal bobbins used to be 3 for 49 cents and one day they were 3 for 2 dollars. The manufacturer had a supply and demand monopoly and capitalized on it. Another manufacturer came up with a cheaper bobbin made of plastic and there were clear instructions that the plastic bobbin was not to completely replace the metal bobbin. I am not the only person who knows this.