Can't import Kindle content on Windows 10

I've installed Calibre 5.5.0. All my Kindle content is in a single directory on my Windows 10 Pro PC. Within that directory, each book is stored in a separate sub-directory.

When I try to import the books individually, I find with some titles (presumably the out of copyright ones) calibre shows only a .azw file from the directory and the import works fine. When I try to import other books individually (paid ones) calibre shows a .azw AND a .azw.md file and the import fails.

What am I missing? When, previously, I used calibre on Windows 7 Pro, I seem to recall being able to import all my books en-mass.

Can someone please help?

Thank you.