Regenerate Thumbnails plugin may help you there, fixed similar issue i had.
Although i was uploading via database not directly so it didn’t create them automatically. plus it took me around 7 hours as i have over 7k images :/ i’m sure much faster for you.
Regenerate Thumbnails plugin is not working also. I had this experience since i completely reset my WordPress and uploaded the latest xstore theme and old database. Old images thumbnails are showing, But newly updating images are not showing in Media Library.