If there are no slots available for booking online, don't fret! They hold aside reservations for booking on board. Just go to Guest Services right when you board and get yourself a reservation. It shouldn't be an issue.
We had a situation on our cruise in Oct 2015 where they switched our pirate night for another day and thus moved our BBB reservation to that day as well. Unfortunately, we had an excursion that conflicted so we couldn't go so I tried to rebook it online, but it was totally full for the new day. When we got on board, we cancelled the conflicting reservation and they had plenty of available times for us to move into.