[Starson17]

I think you're just seeing the delay between the time you download a cover, and the time cover browsing mode rescans the directory and realizes there is a cover in there. Shut down calibre, start it up again and see if there are any books that are missing a cover that you think should have one. If the directory in calibre's library has a "cover.jpg" file in it you should see the cover, and I always do. Otherwise, you should see the default cover.

If a book has internal cover art, that image won't show up unless you tell calibre to get its cover art from the book. To do that, open the edit metadata window, select the format with the internal cover art in the upper right window, then select the cover art icon to the upper left of that window. If calibre can read the image from that format, it will extract the image and copy it into its library as cover.jpg.