Just a guess... are you trying for Christmas? It's booked solid from Dec 19- new year's day. I've had the same trouble! I called yesterday after trying for a room for at least a month online and got what I needed. The first time I called, there were no rooms open. I called back a few hours later and was able to book all but the first day of our trip. Called a few hours later and added the first day and now our trip is complete. DEFINITELY call for help! And if the CM doesn't seem like they are REALLY trying to help, call back and get someone else. I could tell that CM #1 was not giving a flip about my reservation which is why I called back soon after I spoke with him. The next 2 CMs were helpful and seemed to try harder to at least do SOMETHING, like book part of the trip, etc with hopes of being able to add days and so on.
By the way, CM#1 assured me that the web would show if anything came open. I checked over and over and nothing was showing up and it never did, even though I called back and it had come open. Don't rely on the web if you really need a certain date!
Good luck!