In Dashboard, for some reason, when I'm creating or editing a Post, the "Featured image" field is gone. Have double checked the "Screen options", the option is not there...
But it's available when I'm editing a Page.
It's strange. I'm using the 320Press Bootstrap theme, is compatible with post thumbnails. Have also included the add_theme_support( 'post-thumbnails' ) in the functions file of my theme.
So I decided to test the plugin "Auto Post Thumbnail", and it works, I see the post thumbnail in frontend. But it doesn't matter if the plugin is enabled or not, the Featured image field is not there.