A book you bought directly from Kobo using the wifi, In that case, contact Kobo.
A book you bought some other way, opened with ADE and transferred to the Kobo over USB? In that case please give more details about exactly what has worked and what is failing. Some people recently reported they needed to factory reset their Kobo (after backing up if appropriate) and reregister before it would work.
A book you got some other way. In that case, more details please.