You don't need to see the WP file to answer "have you checked that the description is in both these locations" which, had I of known, would have been my answer to someone asking the question.

The whole point is that a problem often has multiple possible causes, so it’s a guessing game if you can’t see the WP file! Once we see the file, we’re in a position to point out that some information needs to be included in two places but is currently missing from one of them.

I really don't see how that has anything to do with it

Well, you have just discovered the problem yourself by looking at your WP file and recognising that something was missing. We could not have told you this without seeing the WP file, so if you wanted help, your WP file would have had everything to do with it!!!

Without seeing how you have set up the WP file, nobody will be able to answer your question. Where would we start?

I really don't see how that has anything to do with it but anyway, I have just discovered the problem.
It appears that the page meta tag description has to be entered in the description box under Page Properties > Navigation as well as Page Properties > Search Engine > Search Terms