Ensure you are naming your media correctly so that Plex can identify it and download the metadata automatically -> http://wiki.plexapp.com/index.php/Media_Naming_and_Organization_Guide
I ask because it has more than 19 episodes. Meanwhile, you might try going into the media manager and trying to fix an incorrect match on the series. It may be that something isn't being fed to TVDB right or something is causing TVDB to fail to find the series. Try the link below. Your mileage may vary depending on your server OS.