Originally Posted by Joques
So, it all seems very unpredictable :-(
Uschiekid, no DRM notice there. I've tried both books with and without DRM. And - missing covers is one thing (ok, it's a dealbreaker), but non-functioning books is worse ...
The missing cover could be a problem with the source epub. The covers for the two types are generated in different ways by the device. For an epub, the cover is generated from the first page.
For kepubs, if they come from Kobo, the covers are downloaded from the server. If they are sideloaded, the covers are defined in the OPF using the epub3 standard. Something like the following is needed:
<item href="Images/cover.jpg" id="cover.jpg" media-type="image/jpeg" properties="cover-image" />
The epub3 part of that is the properties. The extended driver adds this. If the cover image isn't defined in the OPF properly, or there isn't one at all, the driver can't do this, and the device can't extract a cover.
If the epub doesn't have the cover defined like this, a conversion in calibre, or adding the cover using Polish Books or the Modify ePub plugin will fix it.