This happened to me last week at the San Jose Marriott, host to an AOPA convention. 4 months before the event, nothing to be found, not even "Stay Anytime" awards, but plenty for sale at $249.

I made alternative arrangements a few miles away for 6,000 points. Just for kicks, I checked 2 days before and got a 20,000 standard award without a problem. I was treated to a jr. suite and given a 3pm late checkout all on an award. Its rare but nice when it all falls into place!

My guess is that the reserved room block didn't materialize so more rooms were available. But at check-in, I heard they were very full / sold out for the weekend, so go figure!