I'm having trouble booking a fare on AA.com and wondering if anyone has any insight.

I'm looking at a flight next year from EZE-LAX. It's a ways out so the fares are fairly similar for a long period of time.
I tried Amex Travel and they were returning a result that was a bit less because it was a mixed cabin option - EZE-MIA in business class and then MIA-LAX in economy.
That led to me to try fiddling with the mixed cabin option because, if I can get on the 77W from MIA-LAX, I'm not too concerned about being in business.
At any rate, I tried searching as a multi-city itinerary (EZE-MIA and MIA-LAX) on Amex travel and the right flights popped up but I couldn't book it. It kept giving an error.
I then tried my luck at AA.com and the same options (and some more) popped up but when I tried to book it, it would always error out and not complete the booking.

It seems odd that AA itself would offer unbookable options. Or is it simply a glitch and I might have luck calling in?

Call but when they try to charge you a booking fee, explain to them that the web site shows the itinerary and price, but fails to book it. They should be able to get you to the web desk and book it without a fee if they can replicate what you're seeing.

But sometimes when searching multi-city the search engine will return results that are not really bookable once combined. So a phone agent can for sure price it but the end result may be much more expensive than what was shown initially.

You can also try building the trip in matrix.itasoftware.com. Then copy the JSON booking link and open that link in https://bookwithmatrix.com

Hasn't the telephone-booking fee been eliminated by AA? I'm not seeing one on the current list of fees:

https://www.aa.com/i18n/customer-ser....jsp#ticketing

(Scroll down to "Ticketing and other fees.")

Unfortunately, ITA has same issues as aa.com with multi-city searches and doesn't check married segment inventory (which may not match inventory on individual flight segments).

I tried calling but the agent didn't seem too eager or clued-in to be able to offer any assistance.
I can see it on Matrix and when I do "Book with Matrix," American is one of the options. Of course, once I get to AA.com, I get to the same stage and it cycles again and again. Oh well, probably too good to be true.

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).

It's not working with a one-way search - I've tried using the advanced routing/faring codes to see if I can replicate it, so the married segments issue must be it.

I'm looking at late-Aug/early-Sep from EZE to LAX. It seems the lower fares are via MIA. I was thinking I could shave off about $400 by doing a mixed itinerary where the EZE-MIA leg was in J and the MIA-LAX leg was in Y, especially if I could get on AA1247, which is operated by a 77W. That way I'd still be in premium economy.
I started playing with this on AA.com and this little quirk shows everyday - if you select the individual segments, it prices down to $1433 but if you do a one-way search, it's $1800+.

Does AA even allow mixed class itineraries? I can't remember if I've ever booked one on AA.com. It's fairly simple on other airlines - I've done it several times on BA.com

There are a couple of reasons that I can think of with AA pricing. One would be married segment availability - the multi city search will check availability per segment rather than the through availability. Another would be fares. The fare A-B + a fare B-C may come in at $1400 , but the published fare for A-C is $1800

Perhaps make a telephone call to American Express and see whether an agent can explain why it is erroring out.

I�m finding that a particular fare appears, starting in Google Flights, when I specify a muticity search SEA-PHX/PHX-JFK but the lowest fare that appears for that cabin when I search one-way trips SEA-JFK and select the same flights is $50 higher.

It appears once I�m transferred to AA�s site and pick each segment sequentially but then it won�t let me hold it: website goes back to the search and says the fare isn�t available. Is there a way of forcing the lower fare to go all the way through to letting me hold the ticket? I�ve tried different points of sale on ExpertFlyer but nothing seems to work.

This happens all the time...if it isn't available as a one-way, it won't work due to married segment availability... always frustrating.