1. Check the document’s body text.
Are all the headings numbered correctly? Are they in the correct order from Title 1 to Title n? Is the outline structure correct and properly formatted?
2. Check the heading styles.
Are the headings in the correct style for the document? Are the headings appearing outside/below the table of contents?
3. Check the Outline Options.
Are the various outline levels correctly configured? Are any headings hidden from view?
4. Check the document itself.
Is the document open in any other applications? Did you save it as a different file type?
5. Check document properties.
Are all the document properties filled-in correctly? Are any scripts or macros enabled?