Plex not detecting all video files - help please?

For Plex to recognize these names with any consistency, this will need to be:

TV/
     South Park/
          Season 01/
               South Park - S01E01 - Optional Episode Name.avi

If you use any other format, you can expect issues such as you’re currently seeing. I think technically you can leave the show name out of the file name, but the SxxExx piece is mandatory.

If you have a lot of files you need to rename, a tool like Filebot can be very useful. And once you have everything renamed properly, you may need to perform the “Plex Dance” to get things recognized correctly. This is particularly true if they were matched as duplicates of other episodes or a different series entirely: