If your bathtub refuses to drain despite no visible hair clog in the drain basket, the issue is typically caused by an obstructed roof vent stack or a partial blockage further down in the main branch drain line. When leaves or debris plug the roof vent, air cannot enter or escape the plumbing system, creating a vacuum lock that severely restricts wastewater movement. Verification: Check if other plumbing fixtures gurgle when the tub attempts to empty.
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