Keep checking. I've seen reports from a few different posters that more time slots opened closer to their sail date. Also check once you board. There will be a time and location listed in the Navigator to go for tickets.
There are usually some princess meet and greets that aren't ticketed. Which ship are you on?
ETA: Below is a link to the Navigators from a 3 night on the Dream. Character appearances are listed on the last page of each Navigator. Day 1 on this cruise had Tiana and Rapunzel. And there's something listed as "Princess Appearances" on the last night in the D lounge. I haven't been on a cruise that has had that so I'm not exactly sure what it entails. Lastly, on the last night of the cruise there is something called "See Ya Real Soon". All the characters come out, and there will be a brief period of time when you can meet characters.
http://disneycruiselineblog.com/201...bahamian-cruise-port-canaveral-march-24-2017/