It was reported at this thread that someone was able to secure an upgrade outside of the window.

http://www.flyertalk.com/forum/showt...68#post6612868

I assume that it was not one of the fares that entitle to instant upgrade confirmation at time of reservation or the poster would have done so and did not have to wait for the upgrade window to open.

This may be one of the potential causes. Someone might have rung before the window opened but the agent agreed to process the upgrade.