I don't know exactly. I just asked her to confirm the details of my itinerary and she immediately noticed that something was different in the ticketing of one of my segments and put me on hold while she fixed it.
For the first three, I explained that I couldn't access my itinerary on the web and they said that they could see it just fine and somehow implied that it was operator error on my part.
I did notice that the AA android app pulled up three of my four segments and did not show the problem one, so that could be a clue and you could ask the agent to double check whether that particular segment was ticketed correctly. It's a roll of the dice whether you get a lazy agent or a competent one.