You have to work on the presser foot bar from behind the face plate. There is an oil point in the shaft (black part of the body) where it moves up and down that ofte get stuck. Unscrew the presser foot tension screw on top of the machine; be careful not to loose the small washer on top of the spring, work a bit of oil down there to and in the threadings. Any grime and rust that migth be down there will loosen and in the next few days all parts could benefit from a good cleaning.