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 Connect App, select 'Restart UPnP'. This option is located in the 'Player menu' under 'Discovery options'. The media should now show as expected.