Header outline not displaying as expected

Hello.  I have several pages that I would like to analyse in terms of
heading hierarchy.  The Nu HTML Checker normally returns an outline of the
<H> tags used on the page, but with the pages in-question, that outline is
missing.  Does someone know why this might be the case?

 

All of the pages I would like to analyse belong to the same site.  Here
<https://www.horizonhomes-samui.com/about-us/>  is one of those pages.  None
of the pages in-question begin with an <H1> tag-they all start with the <H2>
tag.  I carried out a test where an <H1> tag was placed *below* the <H2>
title, but the heading hierarchy from Nu HTML Checker was still missing
when I analysed the page.

 

I have confirmed that all necessary boxes are ticked when I run Nu HTML
Checker.  And when I analyse pages on other sites, the heading hierarchy
appears as expected. 

 

Thanks in advance.