Also, doing a sniper game might use different mental faculties than dual n back.

Right. tDCS has a number of parameters you can vary, of which the most obvious are location of electrodes and polarity of electrodes. We were trying to stimulate the dorsolateral prefrontal cortex, done by running a current across two scalp locations; the article mentions temple and arm placements that I don't think I saw elsewhere in the literature (but have the equipment to try that out, so I think I shall!).

The polarity is the other huge thing- if cathodal stimulation lowers the polarization and that helps, then anodal stimulation should raise the polarization and that should hurt. (One of the early proposed therapeutic uses was putting the brakes on brain areas that were overactive in negative ways, like anxiety regions.) It's very possible that we screwed up and misread the paper and applied the voltage in the wrong way, and if we swapped it around it would help.