I understand your frustration. If you can be flexible with your dates you'll have a better chance of good rates. Of course, not everyone can do that.
I'd suggest that you
(1)Keep calling every few days.
(2)Make a list of all possible dates you have available and if there are any "codes" that fit those dates.
(3)Ask if there is ever a "Mississippi" special rate during the year. Ask the CRO to search for it. Find out the dates because you may find that it's during a time you could go.
(4) My best recommendation is...Ask for different lengths of stay. ex: you plan on staying 7 nights. Ask for 7 nights. If it is not available. Ask for any number of nights (2/3/4/5/6) with various arrival dates. (4 nights with the arrival date being 3 days after you arrive in the area) You may have to split your stay b/t resorts, but it could save you $$$ in the long run. Plus, it's fun to try different locations. At this point, be direct and tell them what you are looking for. If the CRO seems clueless, nicely say thank you and call again. Some are extremely helpful and know the registration system well so that they can hunt for things and tell you what you are eligible for.
Good Luck! I too hate to pay rack rates when there are special rates available floating around that I can't get hold of. Again, Good Luck and keep trying various things.