iBooks wont sync with ipad/iphone properly

I have transferred all my pdfs from Dropbox to iBooks on my iMac. The iMac sees them OK.

The iPad and iPhone wont see them - after a while it sees 3, now 10. This after two weeks.

Latest version of OSX, of iOS and iBooks.

I have tried turning on and off each of the hardware, turning iCloud sync on and off, quitting and reopening iBooks.

I have followed all advice about Settings and so on (Sync Bookmarks and Collections), using iCloud happily (option for iBooks turned on).

Pages, Numbers, Photos, everything else works a treat. If only you could see them in iCloud Drive!

UPDATE: I notice two pdfs ARE in iCloud Drive - under Preview and it says "in iBooks".

This when viewed on iPhone.

On the iMac it says Preview is empty.

Where are the rest and is it possible to actually check that the pdfs are actually in iCloud somewhere? It is not clear on my iMac that they are actually in the iCloud, except that the odd one occasionally appears on my iPad!