The EPUB BOOKS cover are 2 files (a HTML and the Image that goes on it) inside that are flagged as 'cover'. It is the flag, not the names.
But I think you refer to the one shown in Details / cover browser.
(the first on the QC check list and again in the Missing check?). That is easy: There is no
cover.jpg in the title folder (or it was not added USING calibre)
it might be in the book, but was not properly identified, thus failing to extract when adding to Calibre . View the book and repair it if needed, then use the 'cover icon on the left side of the formats box to extract a copy for the library view