Yokoyama isn't taking the right hint, she obviously likes him.
What he can't understand is that, for her, that's not a reason to start a relationship, that's a reason to tease him endlessly. Think the big forehead bitch Takagi. What she expects from him is his awkward reactions, which she finds cute, and possibly even expects him to try and tease her back, which she finds fun.
If he properly confesses and they become a couple, she knows he won't be flustered around her anymore and will act as if her teasing is natural, which to her makes these moments lose their charm.
You can see that every time he's about to ask "Do you like me?" or say "I like you!" she conveniently stops him.
This manga will probably stretch this "subversive teasing dynamic" phase until its very end.