Incorrect thumbnails or no image when sharing/liking posts

We seem to have constant issues with this, perhaps somebody can help.

For some reasons the SHARE Or LIKE button don’t pick up the image from the post.

We only have one image per post.

It offers 3 images, all nothing to do with the post itself.

Tried setting the post image as a featured image, no joy.

Sometimes, despite an image being in the post, it can’t find Any image at all, from LIKE or SHARE.

From the flip side of the coin, if we cut and paste the link into a FB post, again it doesn’t find an image in the post .

Any ideas?

I will show the blog if that helps..