Doing a search on AA.com for those dates only gives connections which, I would think, would be more expensive than using AA miles (though you may still want to use Avios or only have Avios). They have to search segment by segment and with the prime flight numbers (AA or Alaska), or at least they did when they were having problems earlier so you have to give them the routing and the flight numbers (i.e.SLC-DFW AA 2195 connecting to DFW-JFK AA 1149 or whatever). Assuming you did this it just seems like the agent really didn't know what he was doing, since any milesaver flight should be bookable with BA Avios.