The problem is that the latest versions of the Amazon reader for PC and Mac don't download the books in a format compatible with Calibre's DeDRM scripts. This is compounded by Amazon flipping off the choice to always update the software automatically (yes, it's stated as "always update automatically" if checked), so the first thing you have to do when starting up is go into settings and make sure the checkbox is cleared.

The web based reader will refuse to display some books, unfortunately.

In my case this was the sort of book than can be only realistically read on a big screen tablet, not on a Kindle.

Well, I've shrugged and downloaded the pdf from libgen

Still useful - thanks!