When adding new files to a NAS drive/media server, that server needs to refresh its media content before it can see/share any new information. This is usually done automatically by the media server at a default refresh time (e.g. every 10 minutes).
It is possible to adjust this default refresh time on some media libraries, however, for an instant library refresh you can restart/refresh the media server via its own web admin interface - Please review the operating instructions for your NAS drive for instructions. For large libraries, this could take some time.