How can I repair a sink stopper that is not working properly?

To repair a sink stopper that is not working properly, you can start by removing the stopper and cleaning it thoroughly to remove any debris or buildup. Check the linkage and pivot rod for any damage or misalignment, and adjust or replace them if necessary. Make sure the stopper is properly connected to the linkage and pivot rod, and test it to ensure it moves smoothly up and down to stop the water flow. If the issue persists, consider replacing the stopper with a new one.