The featured image function will not work if you want the post to be in the Gallery or Video style. If you want that, create a new post, and upload the image(s) you want to be on the front page. You must upload the images and not simply add images that are already in your media library. Then hit publish. At that point, you can delete the images that you just put into your post, but they will still remain on the slider on the homepage. Any other images you upload (not just insert from your library) while writing the post will be added to the slider on the front page.
The featured image is only for Standard-type posts.
Hope this helps
