Please show your file names and folder structure. https://support.plex.tv/articles/naming-and-organizing-your-tv-show-files/
Check the file name extension if you have MP4/m4v. If you do, use a metadata editor app to check embedded metadata for correctness.
Do you have “automatic” or “periodic” library updates activated? If you do, disable them. Then perform the Plex dance with the affected series.
