@Nalapombu - downloading the cover updates Calibre's database, but not the Book. If you upload the book to your Kindle it should take the new cover with it. To update the Book on your desktop/laptop do a MOBI to MOBI conversion.
Here's a discussion between BeccaPrice & kiwidude that should help explain, it starts at post 5 in this thread ===>>>
https://www.mobileread.com/forums/sho...+MOBI+metadata