A sink's stopper may function for years without incidence, making it something you do not think about often until the stopper does not work as it should. When the stopper appears to be stuck, the problem may not be with the stopper at all but with another part in the stopper's mechanism, which lifts the stopper up in the drain and allows it to drop into the closed position.
Checking for Clogs
A large enough clog in the sink's drain located at just the right position in the drain will keep a pop-up stopper from opening or closing, so it will be stuck in only one position. To check for a clog in the sink's drain, you must remove the stopper if at all possible. With stoppers that are stuck in the open position, you can usually just lift them up and out of the drain opening. You may need to unscrew the retaining nut on the sink's drainpipe and remove the pivot rod, and then pull out the stopper to check for clogs. If the stopper is stuck in the closed position, examine the drainpipe from the opening the pivot rod goes into.
Broken parts in your sink's stopper mechanism will cause the stopper to become stuck and unable to move. The stopper itself may be broken, but you must remove the stopper to find out for sure. Once you have pulled the stopper from the drainpipe, look at the hole in the bottom part of the stopper for signs of damage, including cracking. The pivot rod may crack or break, which is the part that actually pushes the stopper up in the drain. Both parts can be replaced easily if you do find damage.
A certain screw in the stopper's mechanism may become loose, affecting the performance of the stopper mechanism so the stopper becomes stuck in the drain. When you pull up or push down on the lift rod at the top of the sink's faucet, it should not slide around under the sink without moving the clevis strap as well. If you do see the lift rod moving without the clevis strap, tighten the screw in the "C" shape portion of the clevis strap.
Readjusting the Stopper Mechanism
Sometimes the problem of a stuck sink stopper comes down to the stopper mechanism not being adjusted properly. Sink stopper mechanisms, like other mechanisms, can come out of adjustment over time. You may notice the sink's stopper no longer keeps water in the sink as well as it did before, which may also be due to it coming out of adjustment. With the stopper in the closed position, you only need to loosen the screw connecting the clevis strap to the lift rod and wait for the mechanism to settle for about one to two minutes before you tighten the screw again.