Have you tried ctrl--shift click on the master page items?
".. The page numbers on the C master pages don't change .."
Better make sure these are "page number codes", rather than 'typed in' -- = fixed -- text. You can check this by drawing a small temporary text frame and inserting a Current Page Number code in it. Does it reflect the wrong number, or the correct one?
Another possibility is that the number does not change because you set it to start a new section. Check the page in the Pages panel -- does it have a small black triangle at the top, just as the very first page of your document has? (Which is there because the first page of a document always also is the start of a new section.)
Thanks for these suggestions.
Yes they are page number codes, not numbers simply typed in. Nor is this part the start of a new section. I was able to get around the problem by deleting and then inserting several pages. It reinstated the numbering, but doesn’t give me confidence that the auto numbers are working correctly.