I suck at spy, it seems like when I go for a backstab 99% of the time the enemy player knows there is a spy behind them and will do a full 180 degree turn just to kill me. This even with silent decloak (Saharan Spy) and nobody else around. Get within 10 - 20 feet of a Heavy or Pyro is a guaranteed death for me. I've ran into a Pyro and even though I am about what seems to be 10 - 15 feet away the Pyro always manages to kill me within 2 - 3 seconds with it's flamethrower/backburner/degreaser despite me backtracking to escape. Same with heavy, being within what appears to be 10 - 20 feet I am dead within the span of 2 - 3 seconds. What really makes it frustrating is that it almost always doesn't work that way for me when I'm playing as those classes. When I am playing as a Pyro I have to be right on the heels or toes of the enemy player to ignite them and it usually takes longer than 2- 3 seconds, same with playing as the heavy.
Also with the sniper I am aiming right at the enemy player when I shoot and sometimes the hit doesn't register. My mouse doesn't seem to be very precise though, sometimes when I try to aim I over-aim and miss the shot.