If some of your photos aren't showing up in the Chatbooks app, start by updating the Chatbooks app to the latest version from the App Store.
Why aren't all my photos syncing?
Many iPhones are set to Optimize iPhone Storage to save space. With this setting on, only your most recent photos are stored on your phone. The full-size originals of older photos are stored in iCloud, so Chatbooks can't pull them into your book.
How do I fix it?
Open your iPhone's Settings
Tap your name at the top
Tap iCloud, then tap Photos
Select Download and Keep Originals
Once your photos finish downloading, open the Chatbooks app and add them to your book.
You can switch this setting back after you've finished your Chatbooks. The photos already in your books will stay there.
Watch how it works
Using an Android phone? See How do I get my photos to sync? (Android)