Similar situation, flight cancelled and rebooking on the app/website would only show me limited options when I knew there were more flights that day (with available seats). Calling AA did nothing (23 hour wait with EXP) so I double book myself by purchasing the flight I wanted on a separate reservation. When I finally got a call back (I was already at my destination by then) the lady said try using twitter. Sucks because I don't have one, but sucks even more because an AA representative suggested I complain on Twitter to get any response at all. They did refund me on the double booking though...so at least they did that.