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If your sink has a pop-up stopper, a simple lever mechanism connects the knob on the back of the faucet to the stopper. If nothing happens when you lift the knob, the mechanism has probably come apart, and it could be because either the vertical or horizontal rod is too short. They are joined by a piece of metal strapping called a clevis, which you can manipulate to compensate for the short rod. While you're working under the sink, take the opportunity to make sure the horizontal pivot rod is properly installed and that the drain stopper isn't broken.
Clear the shelves under the sink and get a flashlight. Although the adjustment isn't difficult, you have to be able to reach behind the sink and see what you're doing.
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Test the operation of the horizontal rod by lifting and lowering it while someone watches the stopper. If the stopper doesn't move or moves less than it should, the horizontal rod might be misadjusted, the end of the stopper plate might be broken or the drain might be excessively clogged.
Unscrew the pivot rod with adjustable pliers and pull it out of the drain tailpiece. Pull out the stopper and inspect the end. If the plastic is cracked or broken, replace the stopper. Rotate the pivot rod while you reinstall it until it catches the stopper plate, then screw it in.
Unscrew the screw that holds the clevis to the lift rod and slide the clevis down until the pivot rod fits into one of its holes when the end of the rod is down. If the lift rod is short and you can't slide it down far enough, replace the clevis with a longer one. You can buy replacement clevis plates at hardware stores.
Insert the end of the pivot rod into the most accessible hole on the clevis, slide the clevis toward the front of the sink and slide a spring clip onto the rod behind it. The spring clip is a U-shaped piece of metal with two holes large enough for the rod. It slides along the rod when you squeeze the ends of the "U" and locks onto it when you release them. If the pivot rod is too short, the clip may not hold the clevis, and it may slip off. You should be able to fix this by adjusting the clevis.
Lower the clevis on the lift rod until the pivot fits comfortably into the hole above the one you were using. Bend the clevis at that point with a pair of pliers.
Insert the pivot rod into the hole you were originally using, slide the clip behind it and test the stopper a few times to make sure the rods stay connected.
The screw holding the clevis to the lift rod is designed to be hand tightened, but it will almost certainly slip unless you tighten it with pliers.