Just to add, I've just added 3 posts with 'WordPress' as the category, one which was copied, one without the image, and one with just a title and one word as the post. All 3 posts with just 'WordPress' as the category are not displaying any image. I'm really unsure as to why this is not working now.
Take a look here; http://web-tricks.co.uk/?cat=3
You can see they are all in the same category, some showing featured image, some not.