That's precisely it! Euphoric nailed it. I was reading about the THC receptors, and it said that different people have different sensitivites to the compound. Exposure to THC sort of wakes those receptors up after a few times. But some folks just don't have those receptors as strongly as others. So it simply may not affect them.

I didn't notice anything the first few times I smoked back many years ago. Then I eventually did. I think it must take some time. . . .