As an example, 8 Out of 10 Cats Does Countdown somehow matched to 2013’s Zombieland.
When I go to Fix Match > Search Options, I remove 2013 from ‘Year’ and change ‘Title’ to ‘8 Out of 10 Cats Does Countdown’ but I get no correct results.
Is it because Agent only has ‘Plex TV Series’ available?
If so, how can I fix this?
I made sure that in settings > Agents > Shows that everything was ticked within Personal Media Shows, TheTVDb and The Movie Database.
How exactly have you named/organized those files in your file system (inside the folder linked to your library)?
w/ libraries set to use the Plex TV Series agent you can ignore Settings > [Server Name] > Agents… those settings only apply to the legacy agents which don’t have per-library configurations. You’ll find all applicable agent settings for the Plex TV Series agent in the library configuration (⋮ > Manage Library > Edit... → Advanced; same for Plex Music or Plex Movie agents).
Sidenote: just tested this with a fix-match on my server and Plex immediately found the correct show (though the show was listed under 2012, not 2013)