JeanPierre Before I try that, there's a strange thing that happened yesterday.
I pasted the URL of a random old post of mine into WhatsApp, and the preview showed. So I tried pasting the URL of a more recent post, but no preview showed. Then I tried pasting the URL of the old post which had earlier showed the preview, but this time the preview didn't show!
Does that offer a clue as to what might be going on?