Hi there.
Similar story for us on our Baltic cruise in June. We could only get a late reservation for Manfredis, and that was pretty much it. But...... we were given a mid cruise survey which I expressed disappointment at the reservation system. The next day when we got back from a tour, we had a message on the phone saying we had a 6pm reservation the next night.
I agree that you should keep trying, and also try to book as soon as you get on board, but constantly trying to sort stuff on board cuts into your holiday time. It`s not something I enjoyed at all. And to be completely honest, it`s gradually driving us away from Viking.
Good luck.