Ereader Kobo doesn't recognize drm

Hi,

I recently bought an ebook protected with DRM. I downloaded it with ADE 4.0, and then sent it on my Ereader (Kobo). But when I try to read it on my ereader, an error appears, saying that the document is protected with DRM and that this document can't be opened with my Adobe ID.

This is very strange because I authorized (with ADE) this ereader to use my Adobe ID  several days before I bought this article.

Any idea?

Thanks,

Yann