Then I can't think a reason why a book doesn't show a cover the first time you send it to the device and you get the cover when you delete it and send it the second time, unless it isn't the Kobo reader the one extracting the cover, but Calibre updating the cover in the Kobo database, when the entry for the book is already created, something similar to what happens with series information, only it isn't updated on connection but on resending the book.
Kobo can be a bit tricky to interpret cover metadata. A lot of reading programs use the opf:cover tag (sorry, I'm not in front of my computer so I don't know the exact syntax), but I don't know why, with some books, I need to add some extra piece in the files declaration part of the opf file to get the book cover identified. And that extra piece throws a warning message when I validate the epub with Sigil, although it causes no problems, at least with the reading programmes I have tried: Adobe, Mantano, and Kobo, of course. Sorry I can't give more details on the exact properties I'm using, I found them in this forum, but I need to access my computer to get them.
I'm one of those strange people hoarding books on my ereaders :-). I could delete some of them after reading, but there are a lot of favourites I re-read from time to time, on a whim, and those go to every new ereader I put my hands on, because I don't know when the mood for re-reading is going to strike, and I don't switch on my computer every day so having my books available is important. Besides, my friends and family know I'm a bookworm, and I get asked for reading recommendations, if I carry my library with me, I can give them those recommendations taking a look into my shelves and the book blurb
