As noted upthread, that sort of itinerary is always a bit of a challenge to get a through boarding pass online due to the interface between Amadeus and SABRE. You can either collect the AA boarding pass at AMS, or at the dedicated Flight Conneciton centre for AA at T3. Sometimes AA has staff at the bus rank in T5, but since they doubled the service frequency I'm not sure it still happens or is particularly helpful these days.