Sometimes they need more mental stimulation rather than the physical exercise. We go to an hour and a half nose work class where Yogi spends about 20-30 minutes maybe actively searching and he comes home and passes out like he ran a marathon. That's just an example, but I really see a truely worn out dog after their mind has been exercised. They all need physical activity, of course, but look at it this way - if you keep increasing their physical activity in an effort to wear them out, you end up increasing their stamina and in turn they need more physical activity.
Just like with people, if I walk until I feel like I'm going to pass out today that might be 2 miles, but I do it every day, my stamina will increase until I could walk 5 miles without being tired. But after working an 8 hour day and thinking really hard while sitting at my desk, I'm pretty freaking worn out.