If your sink basin refuses to drain while the visible P-trap and tailpiece pipes underneath appear completely clean and open, the blockage is located further down in the main branch drain line or the roof vent stack is obstructed. When debris clogs the main wall pipe downstream of the trap, wastewater cannot escape. Alternatively, a blocked roof vent creates an air vacuum that halts drainage. Verification: Check if other plumbing fixtures in the room drain slowly or gurgle when the sink fills.
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