Tried searching for an answer, but I'm not really sure what I'm searching for

. I just got a Kindle Basic (7th Gen) and I've been trying to load some books on it from my Windows computer. I've been having issues with the covers not displaying, a similar issue I had using the Kindle Android app in the past. I did some more research on that here and amongst other things, started using Calibre to transfer the books to the Kindle rather than manually. However, I still found most of them didn't display the covers... BUT that is not my concern at the moment.
The problem I now have is that I deleted these from my Kindle, on the Kindle itself. I made some cover changes, then I loaded them onto the Kindle again with Calibre, but the Kindle does not display them (the books) now. Calibre says they are there and they are in the folder where they should be. I was able to load and have the Kindle display a book I had NOT already loaded and deleted. Why will the Kindle now not display them and how can I fix this? I have restarted the Kindle, but that did not help.
Any input on this is greatly appreciated! (It suddenly occurs to me that maybe an ASIN# change would fix this?)
(EDIT: Oh, forgot to mention, because I just remembered... after deleting the books ON the Kindle, when I connected it to my computer I found a bunch of folders still left, so I did some
housekeeping, which maybe I shouldn't have done...

)