Books aren't syncing into collections ( Apple Books on iPad/iPhone)

When syncing new books to an iPad 2 and iPhone 7Plus, the book(s) will appear in the Library, but NOT in their assigned Collection(s).

Books are managed in the Books app on the Mac and placed in created Collections before syncing.

There are approximately 3000 books in the synced Library...can it be overloading the app on the iPad and iPhone?

Side note: the books ARE appearing in the proper Collections on an iPad mini.

This happened before -- maybe a year or so ago -- with no fix available, but it seemed to resolve itself. Perhaps an update repaired a glitch.

Any suggestions/advice/solution would be very appreciated.