There are a variety of possible reasons, but one which you maybe should be aware of, is ADHD. Quite a lot of ADHD folks report that they have an insensitive or even ‘backwards’ reaction to caffeine.
Another thing is that some research indicates that caffeine’s stimulant effect becomes dependency-forming over time, and that for many daily coffee drinkers, their alertness on caffeine is roughly equal to what their baseline level was before they got addicted. So if you’ve been using it on a very regular basis you might find it only gets you back up to ‘normal’ now.
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