Hey, i reckon it's the same as any drug. I mean, some people are more prone to feeling the effects of a drug/supplement than other people.
The only way to find out is to keep trying and compare the outcome with when you weren't taking anything.
But i suppose there's always going to be a slight placebo effect with it because it's on your mind right?

Oh by the way Pete the ones you gave me don't work, everytime i take one.. I never remember anything!