The collection progress page is the best place to see which episodes and shows are incomplete. Here’s mine as a an example: https://trakt.tv/users/justin/progress/collected
Plex webhooks only send new data, this is a limitation of the webhooks. The scrobblers page, apps page, and blog post all have this disclaimer:
Note: We can only get new data from Plex, we can’t send any data to Plex or sync your past history.
I’d recommend this project GitHub - Taxel/PlexTraktSync: A python script that syncs the movies, shows and ratings between trakt and Plex (without needing a PlexPass or Trakt VIP subscription) for syncing existing data, then using the Trakt Plex webhook for realtime watching info moving forward.