I had some similar issues with cover art, and after some tinkering I found out that the Kobo doesn't seem to delete cached cover art for ePub files we add/delete ourselves (dunno if the same hold true for 'official' books from the Kobo store).
So in my case, I tested a few of my ebooks on the Kobo to check for formatting issues before I actually added the cover art to the epub. The Kobo had cached these epubs with no cover art, and still displayed the default power off screen even after I added the cover.
Here's the solution I've found:
Plug in your Kobo, delete the ePub book from the device.
Go to the '.kobo\images' folder, and find the 3 files that correspond to the book your having issued with, they are named like this:
file____mnt_onboard_!!PATH!!_!!NAME!! - !!FORMAT!!.parsed
where !!PATH!! is the path to the epub on the device, !!NAME!! is the filename of the epub, and !!FORMAT!! corresponds to the 3 sizes the Kobo extracts the cover art to, so it can show it on different platforms/scenarios.
Delete these 3 cached files, then unplug the Kobo from the computer. Now load the book back and let the Kobo reprocess and recreate these cached images.