ToC within Style Source (.ind) not seeing selected styles in other Book documents.

I know first question that will come up is whether I have checked the box for "Include Book Documents." I have.

I started with two identical files and removed sections from both, then synchronized, so I don't think it's a Style naming issue. There are no style groups. This is a pretty simple and straightforward layout.

I'm using the same style in both documents. Works just fine for the Style Source document, but doesn't see it in the other document. Could use some help, here.