Lightroom Classic Book module - no images

I am creating a album in Lightroom Classic Book module.  I went away to add more photos to my 'filmstrip'.  Returned to the Book module to find there are no images in the album!  I previously saved it as a Family Album.  The filmstrip shows which images have been used with a 1 against those used and the pages show photo frames, so it seems they are there but not visible.  Can anyone help please!  Thanks  Michael