Not so central

In my first post I described how I thought that Cohn and Jake would be the central characters of the novel. However, I think that I was wrong. While reading past the first 5 chapters, Cohn seems to drift away farther and farther into the background. In the first chapter it seemed as if this story was about him, but over the next few chapters, he seems to slowly transform into a character that doesn't seem that important anymore. He's just a character that is plainly just... there. For example, in all of chapter 4, Robert Cohn is not mentioned one. This is just very different from Chapter 1 when he seemed to be the main character of the book. I think the author did this on purpose and it has some meaning behind it but I have no idea what that would be.