The clerk's attitude and quoted response to you was unfortunate, unacceptable, and very un-Marriott like. Most, but not all, hotel reward programs are capacity controlled, just like airline frequent flyer miles. As has been suggested, utilizing some of the tools at marriott.com may help you find available dates at properties you want, even the one you previously tried. Some properties require award reservations almost a year in advance. Sometimes properties open up at the last minute when they discover they have unsold inventory.