Once you board, make sure to check what time and where dinning requests are made. Go there and ask if your request was granted. If it wasn't there is still a chance they can do it for you. Go early as there could be others with the same request.
One sailing I had made the request but was busy and did not check to see if it was granted. When we went to dinner, the server led us to the table but half way there I realized it was a table for 8. It was going to be 4 couples. I stopped, told the server that it wasn't going to work for me,, not to worry I would just get fast food or room service. He told us to hold up and he took us to the head waiter who gave us a private table at that point. We were very lucky with that and I never forgot to check again.