There is a big difference between Yozora and Sena, Unlike Sena, Yozora can't bring herself to admit defeat and ask for help, so she vents her frustration on the only person that is willing to submit herself to even bad treatment, because she is being treated like a normal person (kind of) and not like some rich spoiled princess. She will keep it up, winning over Kodaka on the way.
Lets sum it up, Kodaka's attention gains:
Kobato - 6 (after all they are siblings so he does pay her moderate level of affection)
Yukimura - 4 (Vassal)
Nun - 3 (adopted sibling of sorts)
Sena - 3
EroGenius - 2
Yozora - 1
No matter how you look at it, all girls in this club have their eyes on him for one reason or another (and 1 trap) but only Sena and Yozora developed romantic feelings for him, Sena is kind of expressing them, and Yozora is simply unable to probably due to their past, and the fact that she was unable to tell him the truth.