Hi Fernando, thanks for your suggestion. I think I might know why this has happened (but not how to fix it).
I’ve been running this website since 2015 and producing posts since then. In the past, I have used other themes and I suspect that there are “legacy” issues that mean older posts don’t behave the same as newer posts.
I think it might need me to go back into the older posts and do some rejigging. I’ll let you know how that goes.
Kind regards, Fiona.