Norwegian's system is a bit particular with times. If you have a dining reservation before the show, that may be the problem. For tables of 4, most restaurants allocate 90 minutes (Teppanyaki is 120 minutes, as are tables of 6+ at any restaurant). If you have a reservation at 6:00, then the system blocks out 90 minutes. This means that the first thing you can book after your meal has to start no earlier than 7:31...after Choir of Man starts.
That's how I've had it explained to me, and, when helping friends/family for our upcoming Alaska cruise, it looks like that's what was happening.