Part of her story I think (one of the "breakpoint" scenes?), see what you need to do to progress through it (might need to advance Onori and Yui's story). It should be a few scenes in the school rooftop during noon so check there if you haven't.
Just keep progressing Yui's questline, you should have hints of how to do it available in the little journal with a heart in the top right side of the game's window.
