@TheSacredSoul: You could of course clear the cover cache (there's a button for it in the 'Tools' section of the KUAL extension), but no, there's no 'let's forcefully process every books in the documents folder' option

.
It'd be somewhat trivial to add, but extremely resource hungry, if there's demand...
Now, on to that specific book! Clearing the cache won't magically make it work

. If it fails each and everytime, and not because of the 60s timeout, it probably means the format is unsupported, or the metadata is broken. Do you know the format of the book?