Walk animation wont play but idle will

Hi!,

I am making a FPS game but my walk animation won't work, when i press W,A,S, or D, my gun just jerks

heres my script(In javascript):

  function Update() {
      if (Input.GetKeyDown(KeyCode.W)) {
          GetComponent.<Animation>().CrossFade("Walk");
      }        
          else if (Input.GetKeyDown(KeyCode.A)) {
          GetComponent.<Animation>().CrossFade("Walk");
      }   else if (Input.GetKeyDown(KeyCode.S)) {
          GetComponent.<Animation>().CrossFade("Walk");
      }    else if (Input.GetKeyDown(KeyCode.D)) {
          GetComponent.<Animation>().CrossFade("Walk");
      }
        else {
          GetComponent.<Animation>().CrossFade("idle");
      }
  }

So, like I said, My idle animation will play, but when I press WASD, it just has the gun jerk, then okays the idle again, even if I hold WASD.

thanks.