Keep calling every other week or so. It will be so worth the effort to visit the Fort at this time of year. For the past several years, we have always made it to the Fort at Halloween,and it has been our favorite time.
Since my daughter is going from a year round school calendar to a regular school schedule, our October Fort days are over for a while. In fact, I am on the phone right now trying to piece together Fort ressies for the week after Christmas. I was running into the same issues as you. At first they said there was nothing availble for the entire month of December, but I kept calling and have manged to get the 26, 27 and 30th. I am trying to backfill the remaining days. I will not give up, and neither should you....
People's plans change, and days will open up.
Keep the faith!!