Coffeine doesn't work for ~10% of the population (like me).

ADDED:

I don't know exactly what's different and to what extent the effect exists but caffein doesn't have significant waking or alerting effects on me. At least not a 200mg pill (corresp. 2 liter coke or 2-5 cups of coffee). These I take to treat migraine where the caffeine does have a very notable effect on me.

"Caffeine has a tremendously wide variation in action," Regestein admits. "The people who say 'I can drink a cup of coffee right before I go to bed and go right to sleep' aren't lying." Hard biochemical research confirms the fact. Carney describes "one common strain of laboratory mouse, Jackson's Lab's SWR strain, inbred since the 1920s, who is just totally immune to the effect of caffeine: there's no dose that will excite him--not 100 milligrams per kilogram, which would be equivalent to 100 cups of coffee in a human.

And the opposite:

Carney points out that if some individuals are not much affected by caffeine, others--some 5 to 10 percent of the population--are hypersensitive to its effects. These individuals are the most likely to succumb to a serious coffee habit, exhibit the greatest physical and personality effects from it, and have the greatest difficulty in finally kicking the habit.

From Health: Does Coffee Make You Sleepy? Researchers now understand how caffeine works on the nervous system. For some, it may cause the opposite of its intended effect. By Roger Downey

See also http://flipper.diff.org/app/items/5455 for migraine and caffeine.