Because Featherweights were made so well the thread gets caught in the bobbin case if you don't hold the thread. It gets caught behind the mechanism. This freezes the needle and cause your problem. You need to put the machine on it's side , remove the bobbin and remove the thread. Sometimes you can't remove the thread without removing the bobbin case. There is a very tiny screw that needs to be removed ( you need a very small screwdriver to do this) and don't loose that screw. You can go to google for "The featherweight 221 factory" and there is a great pdf with pictures on how to fix the problem under "Jams". I've become really good at this because I just can't remember to hold those threads when I start sewing. Hope this helps.