We have found that the easiest way to get your photos to sync is to update your Chatbooks to the latest version.
Most of the time, in order to leave more storage space open on our iPhones we have them set to "optimize phone storage". If this is the case, only your most recent photos are actually stored in your iPhone. The original full-size photos in the rest of your camera roll are not stored on your iPhone but in the cloud. Because of this, Chatbooks cannot retrieve those photos to place them into a Chatbook.
In order to fix this, there is a really simple solution. If you go into Settings, iCloud, Photos, and then click "Download and Keep Originals", then all your photos will be put back into your iPhone. Once this is complete, you should be able to go into your Chatbooks app and pull those photos into a book!
And, even if you decide to switch this setting back after you have completed your Chatbooks, the photos you have placed in your Chatbooks will stay there.
The videos below should help to guide you through the process of downloading your photos!