Can't send latest version to device

[Willy Wombat]

Hi. When I send the newly created MOBI version of a book to my device (Kindle Fire HDX) it shows an older version. I have cleared out all the old versions using Calibre.

Having read a few threads about similar problems, I think that by altering the HTML code I may have produced a bad book that will not transfer. But I need to alter it, e.g. in order to force page breaks. I have also emended the CSS in order to handle paragraphs properly (I don't want to indent the first para of a section). But without an error report I can't see what's causing the problem.