Airlock activity isn't the best indicator of fermentation. It's possible the lid didn't seal tightly enough, the grommet around the airlock in the hole in the lid is bad, or simply the yeast hasn't produced enough CO2 to overcome the ambient pressure inside. Some yeast strains also tend to make the wort carbonated instead of releasing off the CO2, I've heard of that before (though never experienced it for myself yet).
To be sure, you could open the lid just a crack and check to see if there's krausen forming. If so, RDWHAHB. If not, pitch more yeast.