I figured out a 100% consistent tell when this will happen, if you highlight a word that you want to be a heading if there is an extra highlighted space on the right side of it it will have the function of a heading, if that space is missing when it is highlighted it will just have the style of a heading, not the function.



Anyway, I did "fix" this by way of completely deleting the lines, the ones that contain the word and below, then clicking enter to get space at the beginning of the page, and then retyping the word.

There sure is a lot of tiny details that go wrong when you are editing text in ms office...

I am unable to replicate that behaviour using Word's built-in Heading Styles (which are paragraph Styles). I can imagine something like that happening if you're using a custom character Style and the "extra highlighted space on the right side" is, in fact, a paragraph mark, but not otherwise. You can toggle the paragraph mark display on/off by clicking on the ¶ symbol on the Ribbon's Home tab.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]