Hover a photo in the Photos panel (with the filter set to Unused) and two marks appear:
They are instructions for Auto-Layout Extra Photos, which takes photos that were submitted but did not fit on their contributor's page and spreads them across new photo-only pages, evenly through the book. Mark your favourites first, then run it. If a photo has both, the star wins.
Tap Help at the bottom right of any page and tell us what is happening. Your message comes straight to us and we usually reply within a few hours.