One of my headings isn't showing in the table of contents

Is one of your article's headings missing in the table of contents? It may be because you've got a smaller heading above a larger one, like this:

Heading 2

Heading 1

We won't show the first heading in the table of contents because it isn't very good HTML markup and could rffect your SEO. You should always try and have subsequent headings to avoid SEO penalties.

To read more about HTML headings head (pun intended) over to this Mozilla Developer guide.