If your bathroom sink refuses to drain even though you have cleaned out the pop-up stopper and found no hair clog in the trap, the issue is typically caused by a malfunctioning pivot rod linkage or an obstructed roof vent stack. If the pivot rod fails to lift the stopper high enough, it acts as an invisible dam. Alternatively, a blocked roof vent creates a vacuum lock that impedes wastewater flow across household fixtures. Verification: Remove the stopper entirely and test if water flows freely down the open drain.
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