When you hit a player, and the hits register on your device, but he doesnt go down, its lag.
See, for a kill to register on your device, a signal must travel from your device, the one shooting, to the server, to your opponets device, the one being shot, and thats when he dies on his device. His device has to then signal the server, and the server signals you. Thats when he dies on your device. If either one of you has a bad connection, it will take a while for the hits to register. They will register eventually, though. Never put more than two bullets in one guy, cuz they will eventually register, and theyll go down. Could take two seconds, could take ten minutes, but it will happen. Trust me, bro, he didnt hack.
Get it now?