We visited Oahu last October and I checked online for Pearl Harbor Memorial tickets for several months before we got there. There were never any available, for any of the day of our visit. It just didn't seem possible to me that there were NO tickets available for several weeks in the middle of October.
Showed up around 10:00am on a Saturday morning to the ticket office, and what do you know? Tickets available for virtually every tour group that day. We (party of four) got on the 10:30am tour group, so didn't even have to wait.
I hope the online pre-book works for you, but if not, give just showing up a try. The earlier in the day the better, I am sure.