Here’s what I get now. It’s still not as I expected - probably things are simply full already. On the assumption that it must be something on my end, I tried several browsers on Linux, Mac and Windows, but got the same result each time. Should I assume that some days are simply full?
The Marriot gives rooms at the correct rate for checking in on the 21st, but if I select check in on the 20th, then it reports no rooms are available. Perhaps the first day of ROSCon is widly popular and the 2nd day isn’t? 
The Fairmont Pacific Rim still is not available on the 20th. I guess that must be the expected result.
The Fairmont Waterfront has rooms but still does not give the discounted rate.
The Fairmont Hotel Vancouver is working fine.