If you are trying to construct an itin hop-by-hop using multi-city search in each direction of travel (or a one-way direction), it won't necessarily check married segment inventory availability until all segments have been selected in each direction. I don't know if AA flight search does that, but Google Flights searches does exactly that when using multi-city to construct an itin hop-by-hop (at least it does that with Delta flight searches) and you can see the fare change if married segment inventory differs from individual flight segment inventory after selecting all segments. If you are not using multi-city, then it doesn't really make any sense as one-way and roundtrip searches should already check married segment inventory availability in each direction.