HI WKRP and welcome to our community
I suspect you are referring to the Table of Contents and not
the Index. If I'm correct, there are a couple of things you should
do.
Edit your default window. Expand the Windows pseudo folder
and double-click the window. Then ensure a check mark appears next
to the Auto-synchronize TOC option. (It's in the lower left corner
of the dialog)
The other thing to do is to check the settings for the TOC.
Edit the Single Source Layout file. Expand the Single Source
Layouts pseudo folder and double-click the Microsoft HTML Help
layout. Click the Advanced button. Now click the TOC Styles tab.
Ensure a check mark appears next to the Always Show Selection
option.
Give those a go and I'm guessing you will have what you want.
Cheers... Rick