You are sending it via Calibre? Is it happening for more than one book? My suggestion, do another conversion, mobi to mobi. Also check your image size. I usually use whatever downloads when I edit metadata, from Amazon or some other reasonably sized source.
(I love the cover view. So now I only send via Calibre, and never use the wifi, either!)
Anne, Mom to 4 kids & 4 Parrots.