As most series air finales in April and May, many shows are on hiatus throughout the summer until fall. Consequently, we won't have any new episodes of these shows to add to the site or to your Watchlist (although, this varies by show). Whenever we can, we try to post this information in the Availability notes for a particular show on it's show page (e.g. www.hulu.com/family-guy ) - however we don't always get the full schedules ahead of time, and this is not always possible.
The good news is that most shows will be back in the fall, however, if you're curious as to the specific timeframe for when your favorites will be coming back, you can check a site called epguides.com for specifics.