When scrolling through your Notebooks, you might notice that you can only scroll vertically and you can't turn pages/flip through a notebook horizontally. If that is the case, vertical scrolling direction is most 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.