I have found Marriott.com to lie many times about "No rooms available". If you click on "Are your dates flexible?" it will pull up a calendar and prices on each day showing the total amount of a stay starting on any given date for the number of nights you selected in your search.
Just yesterday the hotel I wanted in Chicago kept showing "No rooms available". However, the calendar was completely full (meaning you could check in on any day you wanted to) and I could book a room. I have no idea why the search engine is returning that nothing is available if there are, indeed, rooms available. And the rate it showed me was in line with expectations, not the full rack rate.
PS - different hotel availability will also show up whether you are logged in or not. I find more hotels showing me prices if I'm logged in (Plat Premier) vs if I'm searching without being logged in.
-RM