Media Library not showing some images

Are you saying the issue is sporadic? Do the ‘missing’ images *ever* show up in the thumbnail view?

Be sure to resize and compress your images before uploading – iPhones (smartphones) store images with outrageously huge file sizes.

Resize:

https://www.birme.net/

Compress:

https://tinypng.com

You can also try this:

https://en-ca.wordpress.org/plugins/regenerate-thumbnails/