If i understand this correctly then, none of your posts are using the featured image; instead you have them directly inserted into your post. I did not look at all of your posts but it appears they do not have the “Continue Reading” often called a “Read More” link or button on posts in a blog,. It looks like you have the full post showing instead.
Currently, the theme does not show the featured image on the “Full Post” view, so at the moment, using a featured image is not going to work until I do a theme update.
One option is to insert the “read more” into your post after the first inserted image you have in a post. This would split your post where you have a summary (the first image that is inserted in your post) and then when someone clicks on the “Continue Reading” link, they will see the full post with all the images in the post.
The other option is to use the Image Post format if you want to use a Featured Image. This will show the featured image in the blog and also on the full post view. Any image that is inserted into the post itself like you have at the moment will not show up on the blog, but the featured image will. You will still need to split your post though with the “Read More” tag from the editor.
It appears I will need to do some updates to this theme and will have to do it this week….including updating the tutorials.