Well, these suggestions are all meant to help, but none has fixed the problem.
1. I must (and I do) create the chapter number in a text frame on a master page because I want it to repeat throughout the book. The text frame simply says Chapter and then I insert the section marker. Accordingly, all document pages appear with a red + sign in that spot.
2. There is no access to the Numbering & Section Options in the master page, at least not after the section marker has been set.
3. Back on the document pages I notice that the Numbering & Section Options begins with a greyed-out checkbox for Start Section. So something must be up with that. I just don’t know what it is. The other items all appear to be settable, but I imagine that I first need to set the Start Section.
The only suggestion I’ve found that I can’t actually try is D. Blatner’s above where he refers to the “book panel”. There doesn’t seem to be any reference for the book panel. I do have a page panel, but I doubt that it’s one and the same.