The pop-up assembly on a bathroom drain has several moving parts that wear out over time. If the unit is made of thin metal or plastic that is easily bent or broken, you may not be able to repair it.
If a stopper is loose and won't stay open, tighten the pivot ball retaining nut.
If a stopper is difficult to raise, loosen the pivot ball retaining nut. If that doesn't work remove the pivot rod, clean out the opening in the drain body, and replace worn gaskets or washers. If the stopper does not seat all the way into the drain body when you pull up on the lift rod, adjust the lift rod.
If a stopper does not hold water, remove it and clean the rubber seal. If there is an O-ring, replace it or the stopper.
If water leaks from the pivot rod, tighten the retaining nut. If it still leaks, remove the pivot rod and replace the gaskets.
An hour or so for most repairs
Long-nose pliers, groove-joint pliers, screwdriver
No special skills needed
Make the work area comfortable; place a bucket under the assembly to catch water
Replacement 1-1/4-inch rubber or plastic washers, perhaps new gaskets, perhaps a new drain assembly, plumber's putty
Some stoppers can be removed simply by pulling up. With others you twist a quarter-turn or so, then lift. A third type has a hole through which the pivot rod passes (shown); remove the pivot rod first. Check the O-ring for damage; replace it if necessary.
Buy a new drain assembly with a lift rod that fits through your faucet or buy a new faucet and drain body. Slip the sink flange through the hole in the sink and screw on the drain body. Tighten the locknut, install the pivot rod, attach it to the clevis strap with the spring clip, and adjust.