Actually, the same thing has happned to me on UA and other airlines. If the fare is a "web special" or "internet only" fare, the phone agent can't access it. Also, occasionally I've seen where the website is offering a fare bucket that shows sold out on phone agent's screen. My guess is the inventory isn't updated real-time and has some lag to it.

Only suggestion I can come up with to save money is to cancel your current ticket and get a credit towards a future purchase (minus $50 change fee). Then, book a completely new ticket on the website at the lower fare you found.

The disadvantage is that your money will be tied up with AS, and you'll need to use the credit within one year. This isn't a problem for those of us who fly AS frequently, but may not be desireable for you if you rarely fly AS.