It is likely due to married segment inventory availability if you are attempting to construct flights hop-by-hop with multi-city search on ITA. Do you see same fare on ITA if you search one-way (instead of multi-city) and use advanced routing controls to specify AA itins ("AA AA" will limit results to AA one connection options). A one-way (or roundtrip) search on ITA will correctly check married segment inventory before quoting fares, while a multi-city search will not. There are also those of us with ExpertFlyer accounts that can check married segment inventory on flights if we know the flights involved (like example above).