After using my Kobo for 1-1/2 years, downloaded books to Adobe Digital do not transfer to my Kobo.

After using my Kobo for 1-1/2 years, I have recently downloaded books to Adobe Digital but they do not transfer to my Kobo.  The message I get is " ... the book already exists.."  I have tried "replace file", "skip file" and "duplicate file".  Nothing works.