I realised that I have a strange problem when I try to share a post in facebook. From all my posts, only for the first post I created the thumbnail image shows for when I try to share the link in facebook, via a sharing plugin (i.e. shareThis, or sociable), or even when I directly paste the link in facebook myself. For the rest of posts I created I get no image at all. I should add here that thumbnail image shows ok when I share pages. The problem is only with posts.
I have build a child theme for twenty eleven, and when I removed it the problem was still there.
Then I removed twenty eleven and replaced it with Kubrick theme, and the problem was still there.
Has anyone ever a encountered a similar problem?