I've seen this when style names are duplicated with different capitalization, or there are two copies in different style groups (or in and out of a style group) and both have been applied to different parts of the text.
Thank you Peter, that is true.
In my case I tried to be as tidy as possible.
What is peculiar is that 3 entries in the same document (there are over 40 documents binded in a book for that title) had the same style applied to them, and only one of them (the first to appear in the document), showed up in the TOC.
I then created a new style especially for those entries, and it still I get the same result...
I am at a loss!
Do those headings show up in the doc itself in preview mode?