Player Movement Problems.

Hi, I have a player gameobject that accelerates forward and is able to move up, down, left and right. The only problem I have is that my movement script doesn't allow the player to move up and down on the Y axis very far. I have tried changing the boundary size but nothing works. My code is a modified version of the movement script from the shooter tutorial, here it is:

 class Boundary
 {
     var xMin : float;
     var xMax : float;
     var yMin : float;
     var yMax : float;
 }
 
 var speed : float;
 var boundary : Boundary;
 
 function FixedUpdate () {
      var moveHorizontal : float= Input.GetAxis ("Horizontal");
      var moveVertical : float= Input.GetAxis ("Vertical");
 
      var movement : Vector3= new Vector3 (moveHorizontal, moveVertical, 0.0f);
     rigidbody.velocity = movement * speed;
 
     rigidbody.position = new Vector3 
     (
         Mathf.Clamp (rigidbody.position.x, boundary.xMin, boundary.xMax), 
         0.0f, 
         Mathf.Clamp (rigidbody.position.y, boundary.yMin, boundary.yMax)
         
     );
 }

Does anyone know what is wrong?