AFAIK some people also have a different variant of the adenosine receptor that makes it so caffeine doesn't bind to it in the same way. I recall seeing a study about this pass by, you might be able to find it with some googling.

There's also a question of tolerance of course. If you drink coffee every day, then after a while you build tolerance and the effect becomes much milder. In order to assess whether coffee is effective you would need to take a break for a week. I know that when I have no tolerance, a strong cup of coffee has a very strong effect, I feel an instant mood lift and become much more active, I want to spend energy.