Headings appear in Navigation sidebar but not in TOC

[GrahamW]

I've never had a problem with the Table of Contents tool before, but today it is not working as I expect it to do.

My O365 Word document is formatted with styles, 'heading 1', 'heading 2', and 'heading 3'.

The items with these styles appear in the Navigation sidebar perfectly.



However, they are not being discovered by the TOC, which displays the message "No table of contents entries found".

I just can't see what is going wrong with them. Searches on Google are almost entirely about the reverse problem, where they are included on the TOC but don't appear in the navigation menu.

Any help would be hugely appreciated.
Thanks, Graham