Many bathroom sinks use a lift rod and pivot rod to move the stopper. A rod that slips out of its clip can leave the stopper low in the drain even when the handle is raised. The sink may drain slowly because water has little room to pass the stopper.
Observe whether the stopper moves with the handle
Raise and lower the handle during normal use. Check whether the stopper lifts fully or drops back down as soon as you release it. If the stopper moves normally and the sink still drains slowly, share that detail because buildup farther down the drain may be involved.
Look under the cabinet for moisture
Check whether water appears near the pivot rod connection or trap without loosening any fittings. Photograph the linkage and note the faucet model if it is visible. A plumber can check the adjustment, replace worn parts and test for leaks before putting the cabinet back into use.
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