I've had this happen - if they don't qualify as true stopovers (where you stay longer than the allowed time 4hrs/24hrs), you can't seem to force the AA booking engine to spit them out. If you do a "multi-city" search, you get the message above. I've been successful in using Google Flights to do the search, create the itinerary, and launch the AA booking from there. That seems to work most of the time.