Can't Read Reserved Library Books on my Kobo

When I borrow a public library book to read on my Kobo "off the shelf", I can download it to ADE and copy it across to my Kobo then open it in my Kobo to read. All works fine. However, when I try to do this with a book that I have reserved in the public library that has become available, the Kobo gives me the message "Your eReader is not authorised to open this book". Suggestions welcome on what is going wrong with these reserved books and what if anything I can do about it.