If you are signed in to Marriott.com while checking availibility, your Platinum override may disguise a sold out situation. I had this happen to me just last night when trying to book an award stay (no awards, even stay anytime, but the hotel showed "available"). Call the Platinum reservations desk just as you are arriving at the hotel before you check in to check on actual availability - they will know if the hotel is really sold out or not.

If you feel that you are getting jerked around and there is availability when the clerk says that they are sold out, just look them in the eye and calmly say "you're lying" and then ask for their name as well as for the front desk manager and GM with the reason of "I just want to know who to mention in my letter to corporate...". This has worked for me in the past and usually gets their attention really quick without creating a ruckus.

The former Marriott in Brookfield would pull this kind of cr@p a lot and I was not surprised when it got rebranded as something else.