Your question would be better asked in this
https://www.mobileread.com/forums/forumdisplay.php?f=225
forum about android devices in general (you asked your question in a forum that deals with the Nook readers specifically.
I can answer that for Nook color readers that don't show covers properly on sideloaded content, the program "calibre" is useful in re-converting the book from .epud to .epub and that can often correct the errors that caused the cover not to be displayed.
You can find more about calibre here in the calibre forum as well.