I had this issue a few weeks ago when booking a roundtrip RDU-SIN. I observed AA.com, ITA, my AMEX corporate booking web tool, etc think there is availability at a fare bucket that is not (e.g. think K is available when H is actually the cheapest fare). Interestingly enough, EF shows correct. If you call AA.com, they can book the proper fare via the phone to get all AA flight numbers. This was a business booking for me, and I was able to book via AMEX on phone as well.