You must have at least completed the other routes? For me, the only thing missing was her and Hange route, this game is really complicated and the "quest" mission is never subjective, especially in Chistine's route. try to talk to her every time at evening. try to remove her blanket when she is sleeping at night.
every evening she's talking to Ymir ans isn't interested in talking to me (she can't be clicked), every night I'm pulling down her blanket and i have completed (for now) every other girl (is all stuck on Christa, Elsa, Sasha and Hange needs progress on Christa, instructor on Hange and Mikasa needs to finnish all other "branches first").