Bathtub's that have a built-in pop-up stopper use an assembly that has a large metal spring as part of the drain plug mechanism. Over time, the mechanism will need to be adjusted to keep the stopper from sitting too low or high in the drain. Eventually, you will need to replace the assembly, swapping it out for a new one. Removing the old assembly and installing the new one will take less than an hour's time, even for a novice do-it-yourselfer.
Press down on the drain stopper's lever, popping the stopper up all the way. Grab the sides of the stopper's top and pull the stopper, along with the attached rocker arm, out of the bathtub's drain.
Cover the drain opening with a washcloth, and then use a screwdriver to remove the screws on the overflow drain cover. Pull on the pop-up drain lever to pull the overflow drain cover away from the wall.
Pull on the overflow drain's cover until you have pulled out the linkage system and large metal spring from the overflow drain. Lower the new linkage and metal spring into the overflow drain, and tighten the screws that hold the overflow drain cover in place.
Insert the end of the new rocker arm into the main drain opening. Continue sliding the rocker arm into the drain until you can insert the stopper into the drain opening. Actuate the lever on the overflow plate to make sure the plug moves up and down in the drain opening.