I am platinum and am not able to get into some hotels. The last I checked, I was told that the rooms were not made available at that time. I would call reservations and ask them if the hotel is entiresly booked. They can tell you that. Your booking is getting close, is there another hotel in the area that you could try?
Try this as a workaround:
On the marriott.com homepage enter the city where you are trying to make the reservation. Click 'my dates are flexible'. Enter the first day of the month that the reservation is for....for example, enter Jan 1, 2013 for your reservation. Enter 1 for the number of nights. Click on 'Find' for the hotels in that city. Find the hotel you want and click 'View Rates'. The entire month will show by day and rates for each day will show in the calendar.
If you are booking points, click the 'Use Rewards points-NEW' and then 'Find'. Find the hotel you want and click 'View Rates'. The entire month for points will show by day.
Maybe it will show some availability that will work for you.