I can't shoot back

Ok, so I am relativly new to unity, and I have started a "Top Down shooter" I can move an all, I can shoot in any direction forward, but when I try to shoot backwards the bullet just appears, but it doesn't move. I can shoot up, up-right. right, left, up-left, but any direction that includes down the bullet doesn't move. Please help...

here is the script:

var speed = 6.0; var gravity = 20.0; var bulletPrefab:Transform;

private var moveDirection = Vector3.zero;

function FixedUpdate() { var controller : CharacterController = GetComponent(CharacterController); if (controller.isGrounded) {

     moveDirection = Vector3(Input.GetAxis("Horizontal"), 0,
     Input.GetAxis("Vertical"));


     if (moveDirection != Vector3.zero) {
         var rotation = transform.rotation; 
         rotation.SetLookRotation(moveDirection); 
         transform.rotation = rotation;
     }


     moveDirection *= speed;



     if(Input.GetButtonDown("Jump")) {
         var bullet = Instantiate(bulletPrefab, 
                                     GameObject.Find("spawnFireBall").transform.position, 
                                     Quaternion.identity);
         bullet.rigidbody.AddForce(transform.forward * 1200);
     }
 }


 moveDirection.y -= gravity * Time.deltaTime;


 controller.Move(moveDirection * Time.deltaTime);

}