Is it a problem with the ads, or merely the lack of leverage over the mind of a person out and about in the world, doing things? Perhaps just not doing the things the advertisers wish/. I'm more inclined to believe mobile is harder to crack because people are either hyperfocused (one what is in front of them) or hyper-distracted (by the world around them), in this context. They don't have the bandwidth to deal with "extra".