Where are you trying to go that you can't get rooms? Is it the dates that are causing problems. Have you called Marriott to see if the agents can help? Sometimes you just need to change your dates a day or two.
There are no "black out dates" any more but the hotels generally only have a certain number of rooms available for points, especially if its a busy time or there is an event in the area. Your Plat. room availability is only for paid rooms so you have a few options. First call the plat. line and see if they can help. Sometimes they can call the hotel and make something happen for you or find a comparable property that you can use points at for the same time period. The second is reserve the room at the best rate you can for a paid stay, then convert the points you would have used into gift certs to pay for the room. Depending on the rate and conversion, you could break even.