I too have asked myself this question. I am just guessing, but it may be that people are making reservations over the phone. I have run into this too many times for it to be a computer issue on my side. If it's not that, then maybe their system just defaults to none available right at the 6mo mark when too many people are trying to get on to book - example, 10 people max on their system at any one time, default to nothing available. Try back in a few hours or even a day and see if you still have the problem. That would be my only advice.
