Book cover won't show

I just added several books, and although it shows up in Caliber the cover for one of them does not show up on my Kobo. I have hundreds of books installed on the Kobo and this has never happened before.

I tried removing the book file from the Kobo and re-adding it from within Caliber. No joy. I then tried changing the cover to a different version of the same thing (slightly smaller),and re-adding it again. Again no joy. The cover just will not be displayed, not even with the text for the title as has happened in the past. Just a blank title page, and no thumbnail in the listing.

Anything I can do?

Ron