Cover image on web page not showing in Facebook

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.