Idk why but my liking score doesnt go over 40
It's cuz you didn't read changelog, where dev pointed out that he locked the pprogress of relationship on levels 20 and 40 behind certain special interactions.
Lemme give you a hint, when you start a night you have an option to give Yuna some things, there should also be "chat" option, where she'll give you a tip on what you should do to progress further in relationship.
Cheers