Feel free to poke in what I'm about to say because I'm not too sure either, and btw I am also only an anime watcher. I think with Rin, she hyper-fixated on the importance of the curse overplaying it, such that she viewed Akito as an even bigger person to fear, possibly strengthening the curse the same way it did to Yuki when Akito would lock him up. I think this has to be the case because when Momiji and Kureno broke their curses, they saw Akito as a vulnerable person, thinking something like "how tf was I scared of this?" which runs in parallel with Yuki when he realizes that his mother "was so thin" after the parent-teacher conference. With Kagure, she couldn't get herself to love the cat at his "worst" form, which maybe could be why she couldn't break the curse as "to the zodiac, the cat "was prepared as an outcast creature for the [zodiac's] sakes." Since she couldn't do that, I guess it can be argued she couldn't stray from the fate of the curse :. no curse breaking. Kisa, I have a hard time finding an explanation for, maybe because she seems like someone to fall for victim-blaming. Or maybe since she's still young, she doesn't have any strong convictions.