Libra Colour Dropbox and the Libra - File Limits?

I recently bought a Libra Color. As a part of learning what it does I found that I can use Dropbox to store EPUBs and to sync it with the reader.

So, I sort out a selection of books, copied them to the Kobo folder then sync'ed the Libra with it.

Most of the books copied over, some didn't. And I don't know why.

I deleted the offending books from Dropbox and copied them back over and re-sync'ed. Same result.

Then when browsing the Kobo's various menus I found the Dropbox one. I tapped it, a window opened to display the Dropbox folder's contents. In there I could see the non-copied/downloaded books. I tapped on one and it copied to to the reader.

Is this normal? The books are valid EPUB files and can be read on the Kobo.

Thanks.