Use local thumbnails for episodes

Plex Media Server Desktops & Laptops
13

I recommend you try renaming the files according to the official schema, e.g.

TV Shows
  Phineas & Ferb
    Season 1
      Phineas & Ferb - s01e10 - Lost in Danville - The Inator Method.mp4
      Phineas & Ferb - s01e10 - Lost in Danville - The Inator Method.jpg

Then try re-adding the files to your library using the Plex Dance procedure.

If this doesn’t make a difference you should be able to collect your server logs to see if they show what’s going wrong (either checking them out yourself or sharing them here)