Trying to authorise my new Kobo to Adobe Digital Editions

I authorised my new Kobo and can download library books onto it, but when I try to open them once they are on the Kobo it says it can't be opened because its not authorised. When I look in the settings menu on the Kobo it says it is.  I've tried de-authorising and re-authorising but it makes no difference.  I can still load and open books on my old Kobo.