Most bathroom sinks have a “pop-up” sink stopper that opens and closes when you operate a small knob or lever built into the faucet. Push it down, and the stopper pops up to drain the sink; lift it , and the stopper closes.
Most of these mechanisms need adjustments from time to time, and many are never set just right in the first place. Take a look at the illustration. The rod at the rear of the sink will allow adjustment where the metal strap with holes attaches to the stopper rod. The rod can be placed in holes at different heights to raise or lower the mechanism. The perforated rod also allows a sliding adjustment where it is attached to the solid rod that goes up through the sink.
If the sink is plugged, place a bucket below the sink, then loosen the nut and pull back the rod for the pop-up stopper at the tailpiece of the sink drain. (A little water may leak out.) This will disconnect the pop-up stopper, and you can remove it to clear out debris that always collects here.