Hi,
The watchlist here is a “plan to watch” list. After you start watching the show, it is removed from your list and added to your “continue watching”. If you are up to date and a new season releases, you will see that season in your calendar and the show will be back to your “continue watching” after the episodes are released.
You can read more about how the watchlist works here: What is the Watchlist? What kind of items should I add to it? - Support / Tutorials - Trakt Forums