Make enemies spawn again

Unity Engine
1

Hey guys,

I have a script here that is supposed to spawn enemies after a randomly generated amount of seconds, but it only works once. Here is my code:

#pragma strict
var zombiePrefab : GameObject; //Zombie prefab to spawn
var spawnController : GameObject; //Main controller to dictate how many to spawn
var player : GameObject; //player
var zombiesSpawned : int; //keeps track of how many zombies spawned to report back to controller
var waitTime : int; //how long to wait until the next zombie is spawned
var waitMin : int; // minimum time to wait
var waitMax : int; //maximum time to wait
var spawn : boolean; // tells to spawn or not

function Start () { //what happens at start
waitMin = 3; // 
waitMax = 10;// ^ Sets initial wait times
waitTime = Random.Range(waitMin,waitMax); //Chooses a random number inbetween min and max to set for waittime
spawn = true; // Tells spawner to spawn
}

function Update () { //constantly happening
if(spawn) //if the spawn variable is true
{
	Spawn(); //call function Spawn
}
}

function SetSpawn() { // resets the spawn variable after spawning
	if(spawn == false) // if spawn variable is false (happens after every spawn)
	{
		print("SetSpawn");
		yield WaitForSeconds(waitTime);
		print("Waited");
		spawn = true; // makes it spawn again
	}
}

function NewWaitTime() { //resets the amount of time to wait to a random between min and max
	waitTime = Random.Range(waitMin, waitMax); //sets waittime
}

function Spawn() { //actually spawns Zombies
	if(spawnController.GetComponent(SpawnTimer).roundDone == false)
	{
	Instantiate(zombiePrefab, transform.position, transform.rotation); //spawns zombie at location of spawner
	zombiesSpawned +=1; //adds to spawned count to keep track to send later
	NewWaitTime(); // resets wait time
	spawn = false; //sets spawn to false (to make you wait until you spawn another zombie)
	SetSpawn(); //Runs setspawn to restart the cycle
	
}	
}

What is supposed to happen is it spawns an enemy, then generates a new number and waits for that many seconds, and then spawns another one. I dont understand why its not working, but it spawns one, and then the spawn variable just stays true.

2 
function Start() {
  Spawn();
}


function Spawn() {
   //do the spawn stuff
   ;

   //tell spawn to run again after a short wait
   Invoke("Spawn", Random.Range(minWait, maxWait);
}