I think Kevin Rudd's election hurt them somewhat, he's just too boring to make fun of. Their stalking of Howard was always good for a laugh on the other hand.
Rudd doesn't make it easy for them to get at him. He seems to be constantly all over the world and unlike Howard he doesn't go one nice long walks where the Chaser can pester him. In fact they did that joke the other week, they caught up with John Howard on his walk to annoy him because Rudd isn't as available as Howard was.