What was the cause of Jordan and Liz's falling out in the novel, You Should See Me in a Crown?

Jordan is Liz's former best friend from middle school. The pair had a falling out during their freshman year and Liz believed that he did not want to be friends because he was more interested in joining the popular crowd. Jordan always missed Liz's friendship and sent a letter that Gabi never passed along to Liz. Jordan and Liz eventually reconcile and he is named Prom King alongside her being crowned as Queen.