Certain book covers not showing up

[geekgeek]

I've encountered a strange issue on my Kindle Oasis, not sure if it's related to Calibre.

I have 2 sideloaded books that were not purchased from Amazon. I used Calibre to convert them to AZW3, along with a whole bunch of other sideloaded books. However, these 2 specific books aren't "detected" by my Kindle for the purposes of Goodreads and having a cover. While on standby mode, my Kindle just says "Choose something to read and see its cover here".

What's strange is that it only seems to affect these 2 books, even though I converted them and edited their metadata the same way as all my other sideloaded book.

Any ideas what the issue could be?