Upvote:3
There really isn't a way to find out besides contacting the airline and asking. It's possible there's a payment issue or a delay in coordinating with other airlines (if the itinerary involves segments on other carriers) or just random delays in the reservation system. Without access to the airline's internal systems, we could only speculate.
American suggests you contact reservations by phone. You could also try sending them a direct message on Twitter or Facebook. They can look up the details, see the reasons for the delay, and take steps to get it resolved.