Cannot see some books on my Kindle Fire

I know that this is a post that should also appear in a Kindle Fire forum, but I thought I would try it here as well.
Calibre tells me that some books are on my Kindle, but when I look for them in the regular reader, they are not there.

According to Calibre (and my on-device file explorer program) the books are in the Books directory as they should be. All books are in MOBI, PRC, or AZW3 format.

Any basic ideas on what I should be looking at?

I feel like I should be doing some kind of a 'refresh' on my library of books to update the list.

Thanks all