By default, WordPress displays images that are attached to posts, as well as images that are categorized as unattached, in the media library. It’s strange that your media library only show the attached images.
Perhaps there’s a plugin on your website that alters the list of images shown by the media library. You could test if this is the case by temporarily disabling your other plugins.
It’s a design choice that images aren’t attached to a slideshow after being uploaded to one.