I am currently palying as a DS female sentinel. I had a level 15 and got Visas Marr in my party on Dantoine (2nd planet). I tried speaking with Kreia both outside and inside the Ebon Hawk, before and after speaking with Visas but nothing, I don't get the options for the prestige class as indicated in the Prima guide. I also tried to wait until the next planet and went to Nar Shadaa and the option didn't trigger, even after leveling up???

 

And it's not because I didn't speak with my party members since the beginning of the game: I am constanly checking if they have something new to say and I even gained some influence over Kreia.

 

Am I doing something wrong or is it possible that it's a script that is not triggering?

 

(I have noticed some dialogues branches not triggering at the right moment at a few places in the game but I don't know if this is also the case for the prestige classes)

I've heard rumors that you need to lean at least 75% toward either Dark or Light, but I was already at Light Mastery when I selected a prestige class, so I can't confirm this. How far have you fallen to the Dark Side?

Yes you need to be either 75% dark or light. If you are wavering between a choice you can't get the prestige class at that time.