Thanks KokkieH,
“If you notice a post appearing on Facebook without an image, try running its link through that debugger as well – if it doesn’t show an image there, you can force Facebook to re-scrape the post”
Do you mean to run the facebook url in the debugger, it says that’s not allowed – Facebook URLs cannot be crawled.
Actually found a work around… if the post fails to include the photo, I can go into Facebook published posts and then tell it to refresh share attachment. Not ideal, but at least it corrects the missing image.
Therefore definitely a Facebook problem. I would just love to know why it fails sometimes and not other times. Thanks everyone for the help.