I saw it happen once. I am not sure if ALL of the fare classes were "unavailable" except the most expensive, or just the decent ones, but I remember going back in for my daily check, and it had opened back up. People do cancel and change dates, the only thing I would recommend is that you check every day. If that is the case (cancellations cause a fare to open up) there may be only a few seats at the low price and if you don't grab them quickly someone else will!
Just a thought, have you tried putting in a fewer # of people to see if there are ANY seats available at a better fare? Good luck!