Many times I have not had seats clear at the 72 hr. mark. They usually clear at 48 hrs., or even 24 hours. When it gets closer to flight time, AA can have more confidence that a full-fare F customer won't want to buy them. But during a high demand period like Thanksgiving, they may not clear prior tp day pf departure, even for a PLT.

As others have said, once you get to 4 hours from flight time, control gets turned over to the airport, and you should make sure to request to be put on the airport list.