From the creator KOVIDGOYAL
The NC has a bug which causes it to not display covers if the id of the cover element in the manifest has any value other than cover.
calibre explicitly works around this bug when outputting EPUB.
Found here
https://www.mobileread.com/forums/sho...d.php?t=130230
So the first thing to do if cover not loaded is just convert your epub to epub and the problem will most likely be solved.
Maybe this info should be in the first post of the Sticky