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.
See also http://flipper.diff.org/app/items/5455 for migraine and caffeine.
What do you mean with "doesn't work". Your A1 receptors are formed in a way where caffeine doesn't connect to them?
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.