This might sound insensitive but it's possible you're not aware of the effect. For a while I also thought I was not stimulated by caffeine, but I realized I just didn't know how to recognize the effect.
Edit: I would add that being high on caffeine doesn't necessarily mean I have trouble falling asleep.
Yeah it's possible, but I'm sure it is having some type of an effect. I don't doubt stuff is happening at the cellular/ chemical level. It is just so difficult for me to even perceive. It could be changing my conciousness more dramatically than I realize, but as far as the purely physical effects many people report (becoming jittery/ restless, trouble sleeping) they are just not there for me.
Do you not feel "more awake" with the ability to focus better, etc.? I find that as long as I'm focused on a task the effects of caffeine are negligible; it helps me stay focused. As soon as I go interacting with people, the effects become really visible and a hindrance. My mind is a bit too fast and fragmented for these slow and broad interactions.
I don't know by which method to begin to recognize it. In myself I started to notice slightly more fragmented thought patterns; proneness to having more new thoughts interrupting the current line for thought.
Edit: I would add that being high on caffeine doesn't necessarily mean I have trouble falling asleep.