Take the bezel off (outside ring) and remove the center button - get some soft emery cloth and rub it around the inside of the air button and around the outside of the center button - you may just have some calcium built up causing the button to stick.
If that doesn’t fix your button then you may need a new button. Air buttons are sold by the size of the hole in the tub.
There is a nut on the underside of the air button that holds it in place.
You will need to remove the complete air button, measure the size of the hole in the tub in order to get the correct size for replacement.