Watch this video from This Old House to learn how to fix a bathtub drain stopper.
1. Remove the two screws holding the trip lever plate to the tub wall. Put screws in safe place, so they don't fall down the drain.
2. Grasp the trip lever plate and carefully pull out the brass waste-and-overflow linkage.
3. If the cylindrical brass plunger isn't attached to the end of the linkage, use a flexible retriever to reach down into the overflow hole and grab the plunger.
4 .Once you've retrieved the plunger, hook it back onto the end of the linkage rod and pinch it closed with pliers. Check to be sure the plunger can't slip off.
5. Loosen the locknut and rotate the plastic fitting to lengthen the linkage about 1/4 inch.
6. Tighten the locknut and slip the brass plunger and linkage back down the overflow hole.
7. Replace the screws in the overflow plate, and flip up the trip lever.
8. Fill the tub with a little water, shut the faucet, then check to see if the water stays in the tub. If it drains out, remove the overflow plate, lengthen the linkage a little bit more and reinstall it.
9. Lift up on the trip lever, add some water to the tub and check once again to see if the tub stopper is properly adjusted. Repeat this process as many times as necessary to obtain a watertight seal.