But as macropod mentions are you sure that heading 1 is actually USED in the document.

Plus, just as a wild shot, make sure that heading 1 is checked as the style to use for the TOC. After all you can use ANY style to generate a TOC. I personally never use Heading 1 for my TOC, because I never use the default heading 1 (or 2 or 3) for my headings. I use my own styles. So maybe (again a wild shot) click the modify button and make sure the Heading 1 style is checked as the style to use fo the TOC.

AND that the Heading 1 style is actually used in the document.