It would help if you clarified which steps suggested you have tried. Also, is it a Kobo eReader or a tablet?
If it is an eReader,
- Make sure ADE is authorized with an explicit Adobe ID, not with the 'do not register this device at this time' semi-registration.
- If it isn't, deauthorize ADE using ctrl-shift-D (cmd-shift-D on Mac), restart ADE, and authorize ADE with your Adobe ID.
Actually, this step can be worth trying even of you think ADE is authorized, as ADE sometimes gets it's authorizations confused. - Deauthorize the Kobo using ctrl-shift-E to ADE (cmd-shift-E on Mac) with the Kobo plugged in on USB, restart ADE, and authorize the Kobo with your Adobe ID.
If none of that helps, look at the contents of the Kobo using a file manager (such as Windows Explorer) with hidden files and folders visible and delete the '.adobe-digital-editions' directory on the Kobo.