When scrolling through your notebooks, you might notice that you can only scroll vertically and you can't turn pages or flip through a notebook horizontally. If that is the case, Vertical Scrolling Direction is likely enabled.
From the Document view, tap the More (...) icon in the top-right corner > Scrolling Direction and change to Horizontal.
You can also access Document Editing from the Goodnotes menu in the top-right corner of the Library view > Settings, or from the More menu > Document Editing > Scrolling Direction.