My tub stopper is located outside of my tub on the floor and it doesn't drain. Its in a metal tube next to the bathtub and it connects underground. You have to push the lever down to fill out the tub and pull the lever up to drain the water. When I pull the lever up, it does not drain. How can I fix this?

Yoy take the top off and snbakje from there, Gooid luck, Tom