I just added some featured images to standard format posts, but some do show in the post itself and some don’t (but they do show on the start page). I thought it might depend on the image size, but obviously it doesn’t, for there are images at least 1024×1024 which don’t show in the post, others do.
I also enabled “featured images as headers” in the post information settings.
Is there any specific criteria, which featured images must bring along to be shown in the post header?