Throughout the year, but especially over the summer, many shows go on what's called a "Hiatus" – which means they go on a break and typically don't air new episodes for a few weeks, or sometimes, months. As most series air finales in April and May, many shows are now on hiatus until the fall. Consequently, we won't have any new episodes of these shows to add to the site or to your queues (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.