Originally Posted by
PTahCha
I had the same issue for a flight within 24 hours with 2 seats available, which triggered the ToD upgrade offer. Attempted to buy the upgrade, cleared, and immediately refunded. Called 1K desk, and their explanation was that someone else took the offer at the same time and got the offer instead, and since there is only 1 seat available, the cheap offer is no longer available.
That almost sounds plausible but UA should lock/reserve the offer while payment is made - a common software technique to prevent that problem.
But in my earlier case the offer was available for weeks after my first failure and I tried frequently in that period of time, failures every time.