ball breaks through walls

Hello. I am new to Unity and I am trying to create my first game. The basic idea is that this is a 2D game on a plane like space invaders. I have one enemy that moves back and forth on the x axis and it shoots down projectiles every few seconds (sphere game object). I want the projectiles to be able to bounce off all 4 walls of the game and keep moving in the game space. The problem is that the balls keep breaking through the lower boundary (All 4 walls have a rigidbody) . They seem to hesitate on collision for a second then break through. Is there anyway I can keep the balls constantly bouncing off the walls? (Like Pong if it had multiple balls and 4 walls instead of 2)

I included the code (Javascript) for my enemy's movement, and the ball itself (called enemyblast) Thank you for any help.

Enemy Movement:

var enemySpeed: int; var blast: Rigidbody; static var moveDir = Vector3.right;

//Launches Projectile on 2 second intervals

InvokeRepeating ("LaunchProjectile", 2, 2);

function LaunchProjectile () { var instance: Rigidbody = Instantiate(blast,transform.position, transform.rotation); instance.velocity = Random.insideUnitSphere *10; }

function Update () { var moveIt = enemySpeed * Time.deltaTime;

 //make the enemy move side to side
 
 if (transform.position.x >= 24)
 {
     moveDir =   Vector3.left;
 }

if (transform.position.x <= -24.0) { moveDir = Vector3.right; } transform.Translate(moveDir * moveIt);
}

Enemy Blast:

var blastSpeed: int;

//Makes the enemy fire downward

function FixedUpdate () { amtToMove = blastSpeed * Time.deltaTime;

transform.Translate (Vector3.down * amtToMove); }