Kitchen drain not draining

I have an island sink that's vented with an AAV. This sink has drained fine for 30 yrs but lately it has been real slow. We cleared the trap and bagged the 1 1/2" line. Everything seem to flow fine. reassembled the drain and it's still slow. We pulled the AAV and found that it drained just fine so we replaced the AAV. With the new AAV in place, it still doesn't drain! We remove the new AAV, the drain works fine. We flushed the pipes again through the vent line but still the same result. AAV on, no drain, AAV off, it drains fine. What could be causing this?