epub==>azw3 cannot read book on Kindle

I've been working on a book conversion. I have text I scraped off the web browser screen and have had to hack at the html/css a lot to get a properly working ebook.

So, I finally got the epub looking just the way I want it (without all the extraneous crap on the web page), UTF8 characters all display, lines wrap properly (which they didn't at first).

so then I convert to azw3 to put it in my Kindle for PC (version 1.17), copy it over to that computer and drop it in MyKindle<whatever> directory, open up the Kindle Reader, choose the book. well, to my surprise, I get a proper screen for displaying the book, but it is empty, just gray. the left and right arrows are there, and the various screen headers are there, but nothing displays and none of the buttons change that.

Crazy thing is, this azw3 displays fine in Calibre, and the Calibre conversion log shows no errors.

so I convert epub ==> mobi, and the mobi works fine on the Kindle Reader.

anybody got a clue where I should start?

(I can't really upload the azw3, since I can't distribute this ebook.)

this is done using Calibre 3.15 on Linux.

thanks, in advance!