I fly only AA and have never had trouble redeeming miles.
I do have a working understanding of the airline revenue mgmt systems and Gail answered the question perfectly.
It's a balance of projected flight loads as well as simple supply and demand.
We have all been hearing on the news about the heavy travel season this summer and it should be expected that FF seats will be hard to come by.
Last minute travel is often the only way to get on the most coveted AA routes during peak demand.