When you add new files to a NAS drive/media server, that server needs to refresh the Share folder's media content before it can see/share the new content.
This is usually done automatically by the media server at a default refresh time - say, every 10 minutes. You could if you wish reduce this default refresh time to say 5 minutes from 10, but for an instant library refresh you can Restart/Refresh the media server via its own web admin interface - see the operating instructions for your NAS drive. For large libraries this could take some minutes.
Then, via the Stream Magic 6/Minx Xi/NP30, re-enter the NAS device under 'UPnP Servers' and the media should show as expected.