Hi Kirk, thanks for your comment. I am a tiny bit further... If I select View ==> Enter Full screen, F5 works. Then I'm continuing to rank photos (I use colour rating) and it stops working. Exit Fullscreen. Enter Full Screen. F5 works again. That's my discovery this evening.
I haven't swapped my session out to a different program. The whole time I am in PM, I use dual screens with thumbnails on my left (auxiliary screen) and the full-size image on my main right screen.