Welcome to the Quilting Board!
"Most sewing machine problems are due to the carbon based unit in the chair in front of the machine"
You have to be careful when buying "featherweight" bobbins on eBay. I bought some; although they look identical to real 221 bobbins, they do not work. I loaded one up and tried to sew but couldn't make a single stitch. Upon closer examination, the bobbin extended past the bobbin case ever so slightly...not more than 1/32 inch. Then I switched to the original bobbin that came with my machine and it sewed perfectly. Since then, I found some more original bobbins.
I have bobbins made from the early 1900s that were machined from on piece on a lathe and used in the Singer 9W-7 machines, that work in my FW. I've got original FW bobbins too. We've also got some that did come from eBay years ago and they work.
There is many many variations possible due to tolerance variations.
And then .... here is a reason many things made overseas do not fit American made products properly.
A little story to illustrate my point.
That is part of the problem of using foreign made parts in American made sewing machines. Sometimes the manufacturers do not make them to inch specifications, but they round them to the nearest millimeter. So because of that, a lot of times they are too big.
I chatted with a well-known 221 man from Florida while at Paducah this spring. He says there are NO PLASTIC bobbins made for the FW. That is true. I have searched most of the Ebay sites for some. I was able to obtain 50 metal bobbins from Manny Saler in New Jersey. They have been wound on a mechanical bobbin winder like the one from Wal-Mart and have been working flawlessly. No, they are not a 66, or 15. They are only used on the 221 FW and the 301 Singer. Buy the best you can afford. Never skimp on bobbins. Sew-Classic.com is a good source for good quality at a reasonable price. They ship fast and Jennie knows her stuff in case you have more questions. Good luck.