OP if you're always booking thru Amex Travel, I'd try booking the next flight or two without your DM#, then taking your PNR and pulling up your reservation on usairways.com and adding the DM# a day or two after ticketing. Amex travel has a bit of a delay in committing the ticket purchase; you get your PNR and everything but the reservation is really in a pending state till Amex transfers the funds. Usually this gets done in 24-hours or less but sometimes it takes a while, like if you book on a Friday, it might not be settled till midday Monday. The settling of the ticket purchase is really only required to get the reservation to the point where you can manage it and is probably not related to your issue, but the timing might add to the frustration. At any rate, if there's something weird going on between Amex and US where Amex isn't fully transmitting your DM# or has something else out of whack, letting Amex fully complete their transaction before adding your US# may help get around it.

I've not had this exact problem but have had other problems with US and with various hotel and car rental companies when booking with Amex Travel. It's at least worth ruling out the source of booking as the culprit.

Now, if the same thing happens to you when you book directly on USAirways.com, my theory goes out the window!