This has bugged me for a long time about OneNote: that when I want to add a new set of notes near another container of notes, that clicking in the vicinity of the container will just make that container bigger. This may be one of the reasons that I have not used OneNote as much. Today, I finally did a search about it and there is a solution.
This blog talks exactly about this issue and how you can solve it:
I typically oppose re-blogging or kicking back someone’s blog, but this blog post sums it up very well.