Hi,
The “Watchlist”, is a list where you should add items you still haven’t started watching. After you start them, they are removed from that list and added to your history. When new episodes or seasons release, the show will show up in your “Continue Watching” section. There is no need to keep them in your Watchlist.
If you have any questions about the “Watchlist”, please check this: What is the Watchlist? What kind of items should I add to it? - Support / Tutorials - Trakt Forums