I'm suspecting a weak spring in the tension assembly. If not, then gunk in that same area. Try flossing it with unwaxed floss before taking it apart; that might take care of it. And take pictures as you take it apart. Ask me how I know.
My Featherweight likes to get the thread caught around the spool pin and cause problems. Is that happening with yours?
You didn't, by chance, put more than one drop of oil in the bobbin area, did you? I did that once and got the same problem. That was not on a Featherweight, though.