Downloading library books

Having successfully downloaded a library book which can be read on the laptop (Windows 7) and then transferred it successfully onto a Kobo E-reader, I cannot open the book to read it as there is an authorisation issue.  However, on the Abode Digital web site it is telling me I have authorisation.  How do I get round this issue.  Incidentally I have downloaded books from the same library last year, albeit with difficulty