Can't reorder pages in a photo book using PSE-2018

Once I have over 20-pages of a photo book completed in PSE-2018 it becomes impossible to reorder (move around) any additional pages. I can move the first 20 pages without difficulty and I can move additional pages into the first 20--but can't do anything with pages 21 and up--except move them into the first 20 pages. Can anyone suggest a remedy? Thank you. I'm using a MacBook Pro running Mojave 10.14.3.