Media apps don't look at files (they'd take forever to load if each one had to scan the whole phone for files every time you ran one), they look in the media database to see what's there.
The Media Scan app (which creates the database) runs when you start or restart the phone, when you do a media thing (take a picture, record a video, etc.) or run an app like
media.Re.Scan. It doesn't run when you copy or move files. So either install media.Re.Scan and run it after copying files or restart the phone after copying files.