[Online] Why is the defense (or the player...) so bad ?

Hi guys,
First, I apologize if my English isn't quite good ; it's not my first language :)
Then, I created a team, with a friend, and we regurlarly play online. And we have a problem : everytime the opponents strikers are unmarked or nearly offside (IA is our defense). So, they juste have to receive the ball (from a short or a long pass), to run... and you guess the end :)
We tried different formations, strategies (pressing, offside, counter-attack...), and we even created a new team (in British Islands championship), and it remains the same...
So, what's the problem ? Is the IA bad in defense ? Is there a "best" strategy to avoid that kind of (annoying) goals ?
We tried to take the defense (and IA the attack), but we are always beaten (maybe we are not very good, I guess :) ).
Thanks !
EDIT : I forgot... When we are attacking, we are always interrupted by the opponent's defense. The most of time we can hardly carry on a few passes...|||No answer ?|||You can control the player by yourself? If you see a defender is standing not right, select him, move, and use second man to press.|||dont use 2nd man press, it just takes your defender out of position when the cpu pulls its ping pong passing s**t.
always mark the ball possessor yourself, switch always, you will switch so much the lb button will feel like its ready to fall out the bottom of the controller.
use strafe on defense, the same button as pace control when in possession of the ball, keep a 2 or 3 yard distance from the ball possessor, too close he cuts on you & hes gones.
if you stay a good distance away if he cuts you can tackle him, if he doesnt you can force him where you want him to go.