This is not a Plex issue. It’s an issue with how your files are name based on how these episodes are classified by the metadata sources.
Gods of the Arena is considered to be a special on both TheTVDB (Spartacus - Aired Order - Specials - TheTVDB.com) and TheMovieDB (Spartacus: Season 0 (2011) — The Movie Database (TMDB)).
That means you need this to be under Spartacus (2010) and then under either a Specials or Season 00 folder. You’ll also want to rename your files to “Spartacus (2010)-s00e01-Past Transgressions.mkv” (using the first episode as an example).
After you do that, it should find the series and metadata info without issue.
You’ll probably want to do the Plex dance before adding the series back.
Remove all the related files from any directory Plex is monitoring, scan the library, empty trash and clean bundles (the bundles part might not be necessary) and then you can add the files back.
-Shark2k
