From the settings icon on the web interface try doing Optimize, Clean Bundles, Empty Trash, etc.
Make sure that your media and folder structure follows the naming conventions required for plex: https://support.plex.tv/hc/en-us/art...Based-TV-Shows
TV Shows Have to be like this for example:
/TV Shows/Arrow/Season 4/Arrow - S04E01.mp4
Also check your Agents settings, make sure that theTVDB is turned on and at the top.
Do a full library refresh and it should pull down the info. You could also try turning off other agents to see if they're possibly interfering with thetvdb.
Possibly try turning off "Local Media Assets." Sometimes that helps.