Are you shooting directly at someone?
If yes:
On a multiplayer game such as VC:MP, latency lag (a difference in ping) and framerate lag (a difference in average frames per second, FPS, showed by the game) can make it much harder to kill players.
For people with high pings, or if you have a high ping, you will need to shoot ahead of the player -- shoot where you think they will go next, rather than directly at them. As their ping gets higher, you will need to aim further ahead.
For people who have FPS lag, you need to hold down the mouse button when you shoot, rather than immediately running away. This gives their screen time to process what's going on, and they will see you shooting.
If no:
Then there's a serious problem.
