First of all, I'd suggest reading what your character says whenever you try do gain more relationship exp
If yout skipped text, it's something along those lines: "I feel like my relationship isn't progressing, maybe I should talk with Yuna"
Which points you to the next step - chatting with her, start the night, choose living room, then give gift and "chat" to get a hint from Yuna on what to do next (a hint from me is to go running at night in the city specifically, after you get the info on what Yuna wants)
If all of the above is too much of a hassle, try searching comments or check out changelog on wiki, you'll find some clues or even straight up solution there