Did you include Front Matter in the Book TOC and include a TOC in that?
You need to set up a Book TOC to include Front Matter. If you have an Online TOC you can right click on the TOC name in the TOC panel and select Properties to convert it. Then on the toolbar there is a Front Matter icon.
The process is described in detail in the About RoboHelp sample project. Look in the TOC and Word Output topics.