Ugly Book Covers in Books App on iPad

I have imported some books (not from the Apple Store) onto my Books app on my Mac. In this app, they display the proper book cover (from the publisher). When I connect to my iPad (running iPadOS 13.2) the proper cover (when the books are displayed as thumbnails) has been replaced by the (ugly) generic cover created by Apple. I have tried lots of different things to try and get the proper book cover to display including deleting the book and reinstalling it, running the book through Calibre and updating the cover, dragging and dropping a new cover onto the book in the Book app on the Mac ... nothing has worked. Other books that I imported into iBooks ages ago (so like the recent imports) display the proper cover as a thumbnail so this problem is new. Has anyone else (a) come across this problem and (b) found out how to fix it?

Hope you guys can help?