Originally Posted by
Calchas
AA has a few domestic fares that are only sold on aa.com (particularly their instant upgrade fares). If your multicity involves discounted AA-issued domestic fare components that might be the problem.
Unfortunately this also happens the other way, for example I looked for an LHR-LIN one way fare to position to an ex-EU bargain in September. Needing some AA-coded segments if possible, I looked on aa.com which offered only booking class B on the BA flights at ~GBP 470. BA.com meanwhile had booking class Q for ~GBP87 which was more to my taste. This isn't an airline-site only situation, ITA showed the discount fares too.