Disappearing chapter titles' heading text

[Avraham Makeler]

Hi all,
  RE: Disappearing chapter titles' heading text

I have inherited an FM document, where the some of the chapter titles in the
Book window are blank,

e.g., I have something like:

1 About This Guide
2 Overiew
3
4
5 Main Interfaces
6 Physical Description

The actual 'Heading 1' title of the #3 document is 'Functional Description'.
Why doesn't it appear?

TIA

- avi