Adding cover to book in calibre

So I have been using Calibre for a while to add books to my kindle. Ive recently started having this problem where some of the book covers don't transfer to my kindle. I am using just the [Edit Metadata>Download cover]. After that I just transfer to kindle. Is there a reason why some book's covers transfer and others don't? I do always pick the largest and best looking cover.
Thanks for any help given.