Miriam,
You just reminded me of something. Check the pin that works with the presser foot lever to release the tension. Then check the part with the little bridge across the hole that the pin pushes on. The other thing I did with the Commodore was to flatten the little cross over part back down where it should be. Along with the mangled spring the cross over part had been bent so when the lever was lifted it couldn't release the tension on the thread. I used a 1/4" flat face punch and a hammer to tap it back into place. That made a world of difference.
Joe