I've noticed that if I upload a number of images in media library and add to a media category.
I have a grid with the source as the media category.
Any images in the library that are not attached to a parent do not display in the grid.
I have to manually attach the unattached images to something in WP and then they show in the grid fine!
Multi site user, all latest versions WP, Plugins and theme
Welcome to PickPlugins.
I think you probably want to display media images in a grid!
Please note apart from selecting the post types to Media, you've to also change the Post Status to Any.
Let us know if it helps.