the tiniest bit of MDxx can give the reaction. lets say there was 2mg of MDxx in the pill and your mate was a big bloke. well it's hardly suprising he didn't feel anything, thresh hold dose for most people is 30mg and optimal is between 75 and 125mg.
the test will indicater the presence of an MDxx chemical but will not tell you how much is contained in the pill
now why would people make weak pills? fooling the test is part of the reason, but more likely this pill is from a batch that was repressed from other pills, and maybe those pills were a repress, and so on. we are seeing more of this sort of thing. pills that have been repressed often are poorly mixed, so that one pill might get 60mg but another might get 6mg.
people are greedy, but buyers arent fools. you going to buy off that guy again?