title_body: My HTML toc is not showing up in Calbre MOBI output
upvoted_answer: Just a suggestion but you could try:
  
  
  Outputting your DOCX format to html from within MS-Word,
  Making sure that it all works including the ToC, when it is in HTML format, preferably using more than one browser, and then
  Converting the HTML format to MOBI from within Calibre - 
  
  
  This will save Calibre from having to deal with 2 formats at once, (DOCX &amp; HTML), which might be the source of the problem.