This happened to me on my Kindle app on my Android tablet.
What I did was the following;
1) I "added" the book to calibre.
2) Choose 'edit metadata'
3) Choose 'Download metadata' (button at bottom)
4) Select the links that come up one by one until you see a cover with dimensions of xxx x 500
5) save & convert your book to mobi
6) send to Kindle
Cover will now show. (Go to "all items" on Kindle and all ebooks should show up with covers.