I had the same problem for over a year, After adding plugins, changing thumbnail sizes, adding code to my WP blog, nothing didnt work.
Few days ago I realized that some of the plugins I dont need and decided to deactivate them (share and follow, facebook share)
And now it works everything perfect.
Have a look at the last posts on my FB page
So try to remove FB share or whatever plugins you have, until you find which plugin causes the issue.
I hope this helps most of you guys!