It's a tough debate on what particular time Yamada "started having the feels for him"; some would argue it's chapter 15, others chapter 27, or maybe point at chapter 20, or chapter 4 even. The bottom line is, someone have explained earlier that it was a gradual, little-things-over-time thing. You'd need to read and re-read every chapter/extras/omake to get each subtle interactions they were involved in.