For those who stumble across this topic in the future, here's how I fixed it....
There are actually two facilities for adding a table of contents to the page when you check the box that says "Table of Contents".
The first is a dynamically generated viewlet that is described here:
https://docs.plone.org/4/en/old-reference-manuals/plone_3_theming/elements/visibleelements/plone.tableofcontents.html
The second is a JavaScript-based table of contents and this is the topic I used to learn about it:
Bottom line, you can use either a Diazo style to show the "autotoc" JS-based TOC, or go into ZMI to enable the Plone-based TOC.