If simply updating the Table of Contents (e.g. click in it and press F9) doesn't add your Heading 6 entries, that may be because it's had switches added to limit the number of headings. In that case, try deleting the existing Table of Contents and, in its place, creating a new one via Ctrl-F9 (to create a pair of field braces, thus {}) and typing TOC between the field braces, so you get:
{TOC}
then pressing F9 to update the field. All heading levels should now appear.

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