Books via ADE not visible on e-book

Hi,

I have always used ADE with library books on my Kobo e-reader. Since this year I can't see the books on the kobo anymore. In ADE everything seems to work fine, transfer is succesful without errors. I can see via the laptop that the books are in fact on the device but when I safely disconnect the books are not visible on the e-reader. I tried it with other e-readers and all have the same problem so the issue is not in the kobo. Does anyone know a solution?