WaveSpawn Script. Help

Can someone help me with this i think there is something wrong with the script. It is supposed to spawn 5 enemies in round 1, 10 in round 2, 15 in round 3 etc But when all the enemies in any round are dead (enemyCounter == 0) the script doesn't start spawning again.

 please help.
 
     var spawnPoints : Transform[];
     var zombiePrefab : GameObject[];
     
     var yeildTimeMin = 2;
     var yeildTimeMax = 5;
     
     static var enemyCounter = 0;
     
     var spawnXOffsetMin = 0;
     var spawnXOffsetMax = 0;
     
     var spawnZOffsetMin = 0;
     var spawnZOffsetMax = 0;
     
     var defaultSpawnNum = 5;
     
     var isSpawning = false;
     
     var waveNum = 1;
     
     
     function SpawnEnemies(wave : int)
     {
         var spawnNum = (defaultSpawnNum +  5 * (wave - 1));
         
         isSpawning = true;
         
         for(var i = 0; i < spawnNum; i++)
         {
             yield WaitForSeconds(Random.Range(yeildTimeMin, yeildTimeMax));
             
             var object : GameObject = zombiePrefab[Random.Range(0, zombiePrefab.Length)];
             var positions : Transform = spawnPoints[Random.Range(0, spawnPoints.Length)];
             
             Instantiate(object, positions.position + Vector3(Random.Range(spawnXOffsetMin, spawnXOffsetMax), 0, Random.Range(spawnZOffsetMin, spawnZOffsetMax)), positions.rotation);
             
             enemyCounter ++;
         }
         
          isSpawning = false;
     }
     
     function UpdateWave()
     {
         waveNum += 1;
         gameObject.GetComponent(AI_Damage).maxHealth += 50;
         SpawnEnemies(waveNum);
     }
     
     function Start () 
     {
         SpawnEnemies(waveNum);
     }
     
     function Update () 
     {
         if(enemyCounter == 0 && !isSpawning)
         {
             UpdateWave();
         }
     }