Drain stoppers come either as a built-in piece that you never see, or a pop-up stopper that moves up and down in the tub's drain opening. A stopper will become stuck, either not allowing the stopper to open and let the water out of the tub, or staying open so you cannot fill the tub with water. Stopper mechanisms will come out of adjustment over time, parts will break or debris will accumulate on them, all of which can lead to the stopper becoming stuck.
Remove the Overflow Plate
To get a good look at the linkage assembly that controls built-in and pop-up stoppers, you must remove the cover that sits over the bathtub's overflow drain opening. You only need to remove one or two screws before you can pull the plate away. Keep pulling on the plate until you remove the linkage assembly, and then inspect it for damage or missing parts. For tubs with a built-in stopper, make sure the brass plunger piece is attached to the end of the linkage, or you must retrieve it from the overflow drain using a flexible tool and reattach it to the end of the linkage using a pair of needle nose pliers.
Pop-Up Stopper Removal
If your tub has a pop-up stopper, you must remove the stopper and the arm that is attached to the stopper's underside. Pull the stopper straight up and out of the drain using your hands. If you cannot move the stopper by hand, wrap the jaws of a pair of pliers with duct tape, and then use the pliers to grip the stopper without ruining the stopper's finish.
Clean the Stopper and Linkage
Use a rag to clean off any build-up or hair from the linkage or stopper on built-in and pop-up stopper tubs. Make sure to remove all of the soap scum build-up as well as any other build-up that has accumulated on the stopper or linkage pieces, since the build-up or hair can impede the movement of different parts.
Readjust the Stopper Mechanism
For tubs that have a built-in stopper, turn the locknut on the linkage counterclockwise to loosen it, and then turn the plastic sleeve on the linkage to increase the length of the linkage by about 0.25 inch. For pop-up stoppers, turn the nut on the linkage piece to increase the length of the linkage. Also turn the nut on the underside of the pop-up stopper itself, which will affect how high or low the stopper sits in the drain.