Ok, I figured out the problem. The first leg is oversold, and they haven't released the entire coach cabin yet, therefore I still have an "unassigned" seat. At www.aa.com, even if your seat is unassigned it still shows the seat map (even when there is no seat to actually select). I incorrectly assumed AAmobile would still display the seat map even if you couldn't actually select a seat.

The seat map for the second leg of the trip displays with no problem on AAmobile with the Iphone

Thanks for everyone's quick responses