I had the same experience just now and found this thread.
Had my plans all ready, called BA and got shot down.
It's definitely the computer limiting them as I fed them the flights and they found inventory but when further in the booking they experienced an error.
This was for: Flight operated by SkyWest Airlines as AlaskaSkyWest
- low mileage confirmed on AA and AS sites
- inventory confirmed
- but told could not use Avios on these flights.
- he took the flight numbers, grabbed the inventory but then as he proceeded to the next screens he said he got an error.
I guess no AS as SkyWest bookings allowed with Avios - just curious that there is inventory but cannot seal the deal.