^In a way, the latest chapter explains a lot of that. Shima clearly thinks a lot about Adachi, going in the previous volume as far as to quite uncharacteristically for her normally uncaring self wish for her happiness, but she's extremely bad at starting anything on her own, waiting for Adachi to come to her or contact her instead. In the early days when they both were skipping class, they were naturally together in their hiding place most of the time, which gave them the necessary time to bond. Once they started properly going to class, however, it was for the most part reduced to the times Adachi took initiative to talk to her and make plans for them to be together.