If the Zoom Window is not advancing automatically when you write, it may be because this functionality is likely disabled in the Zoom Window settings.
Enable Auto Advance for Zoom Window
From the Document View, tap the More (...) icon in the top-right corner > Document Editing, and enable "Auto Advance."
If the issue is still there, make sure that you're using the Zoom Window correctly by writing in the blue area that appears in the Zoom Window to move you forward. For more details, please see: Writing with the Zoom Window