It took a while for "The Tunnel" to get released, due to trouble with Half-Life 2: Episode 2's updates to the Source Engine, getting stuck on other things, and generally having difficulty animating. The full story is here.
The series in general. For the most part, one guy does almost everything, including most of the animation. This, combined with bugs in the Source engine that constantly plague production, as well as the production of another, more popular show beside it, has resulted in a mere ten episodes being made since the show's debut in 2006.