I'm finding this more and more with Hyatt:
If you check September 30th for one night there's no availability. However, if you check two nights (arriving 9/30 and checking out 10/2) there's plenty of availability. It seems Hyatt is trying to grab (more profitable) multiple-night stays rather than single-night stays. They're doing this without declaring a "two-night minimum stay". It's simply using their reservation system to make one-night stays unavailable or sold-out.

I've had 100% success in these situations (assuming there's enough time to cancel one night) by booking two nights and then calling Hyatt to drop one of the nights. NOTE: The system will not allow you to drop one night...you have to call.