Plex is picky with TV show naming and organization.
Try naming them per Plex documentation. Plex Dance both series after renaming.
Also, the Plex library must be a TV library and point to the Cartoon Network directory.
Ben 10 (2005)/Season 01/Ben 10 - s01e01.ext
Ben 10 Alien Force (2008)/Season 01/Ben 10 Alien Force - s01e01.ext
See Plex Documentation -> Your Media -> Naming & Organizing TV Shows for details.
