feature picture

Does anyone know why the sharing button sometimes shows the feature image or a content image or sometimes a random image from the site?

I’m testing with the faebook share

This works: http://www.durham2021.org/launch-event/

This doesn’t : http://www.durham2021.org/better-prepared-campaign/

Any suggestions would be helpful, If I can make it work I would be happy to pay the fee for plus but I don’t understand what’s wrong with it?

https://wordpress.org/plugins/simple-share-buttons-adder/