To see the chapter headings in either the navigation pane or the TOC (if one was created), those headings must be created using styles. Because you can see chapters 1 - 12 but, cannot see chapters 13 - 22, I would suspect that chapters 13 - 22 have not been assigned the appropriate style.

Here are a couple of links that will help you:
Numbered Headings
Understanding Styles in Microsoft Word

Hope this helps...