Originally Posted by Dave Noble

That would likely be that EY did not offer fares to that domestic location which allowed travel on an airline that is a partner of AA

Not only does it have to be a valid airline for awards, but the governing carrier has to have a fare that is valid for the itinerary

Taking LHR-ADL as an example, EYs fares require that the domestic sector be on EY ( codeshare ) or on VA . Since VA is not a partner of AAdvantage that will stymie booking the domestic leg