Same thing happened to us when reservations opened for our 10th September cruise. After continuous checking one available reservation showed up - 6pm on the 11th - so we took it. Arrived at Manfredi's - no reservation could be found on the booking system - we were asked had we booked online - and got the impression from the gentleman on the desk that this was not unusual. We were shown to a table and enjoyed our meal. On our way out we showed the gentleman the booking confirmation on my husband's phone. He told us that if we'd like a further reservation to come and see him and in hindsight we should have just decided on a day and time there and then, as we didn't get another reservation. Go to guest services or the desk at restaurant when you get on board - even if, by some fluke, you are able to make an online booking, and try and sort it there.