I exprince that the cover image is not showing when sharing to facebook, when I create a web page with more elements. As long as I only have 2-3 elements (header image, some text and a image gallery), it wokrs and a typical clickable image shows in the Facebook post. Adding more, you only get a grey field like this:
Yes, I have compressed the images, well below 1 MB image size, and it is by adding an extra field (a third or fourth) it becomes useless for sharing.
Also some of the themes even if just adding 1 image, give the same result, no cover image on Facebook, only a grey field. I even had one hour chat support with my, trying different test pages, and it all ended with the same results.
So what to do? If sharing a simople one pager on FB can onle be done with a link and no image post, it is useless for my purposes.
PS. I didn't know that it was impossible to edit a post unless you have posted 1000 x, so I didn't get a chance to correct the mispellings done fast & furious on my iPhone, so please bear over with me.