Someone please correct me if I'm wrong, but my understanding is that with room reservations, whenever someone cancels, the rooms aren't actually "released" until first thing the next morning. (Unlike dining reservations which are available immediately for another caller.) So my advice is to call every morning, early. The more often you call, the better your chance of picking up someone elses cancellation. Personally, I'd be calling every single morning until I got what I wanted, even up until my departure day, and if I still didn't get the room I wanted, I'd make sure to ask at check in, too!