Unscrew the foot-lock drain by turning it counterclockwise. Continue to turn until it is loose from the bathtub and you can pull it out.
Push the stopper down and pull it out again. You can push the stopper out from the inside of the mechanism.
Repeat this until the stopper rises and lowers with ease.
Place the stopper in the drain and turn it clockwise to screw it back in.
Test the stopper by pulling it out and pushing it back in. If the stopper is damaged beyond repair, you can replace it with a new one.
You can do this by unscrewing the old one and fitting a new one in its place.