Impress normal mode PgUp|PgDn don't work?

I am using LibreOffice on Ubuntu 20.04. For some reasons, recently, PgUp and PgDn on my keyboard suddenly stopped working. This makes certain activities more challenging (e.g. creating screencasts). Under certain circumstances, I prefer to be in the normal mode, as the Presentation mode covers everything else, which I also need to use in, e.g. a screencast session. I checked my GNOME keyboard settings. I couldn’t detect any keyboard shortcut conflicts. So, any suggestions to restore the two shortcuts?