Make sure you have the TV Library pointing at the correct location.
As the others have mentioned, naming and structure is important. Do not vary from Plex’s requirements.
The Synology FAQ has good information on media organization.
This works on my DS918+:
/A TV Library <-- Library points here
/Haikyuu!!
/Season 01
/Haikyuu!! - s01e01.mkv
/Haikyuu!! - s01e02.mkv
