Did you calibrate your controller? Also in many cases there is a minimum movement on one of the axes - if you multiply that by 20 you will easily get movement. Per Frame. So consider multiplying by Time.fixedDeltaTime as well.
If you set up your Left stick as an axis in the Input Manager then the values of your horizontal axis range from -1.0 to 1.0. You can check for these values or a little less so the player doesn’t have to whip the stick all the way. Something like -0.5f to 0.5f is what I use. Try this… It basically creates a dead zone in the center half of the stick: (You can of course adjust the values as needed)
function FixedUpdate ()
{
var x : float = 0.0f;
var y : float = 0.0f;
if(Input.GetAxis("XBOX_leftH") > 0.5f || Input.GetAxis("XBOX_leftH") < -0.5f){
x = Input.GetAxis("XBOX_leftH")*20;
}
if(Input.GetAxis("XBOX_leftV") > 0.5f || Input.GetAxis("XBOX_leftV") < -0.5f){
y = Input.GetAxis("XBOX_leftV")*20;
}
rigidbody.AddForce(x,y,0);
}