wave spawner difficulty increase

there's the script

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class WaveSpawner : MonoBehaviour {
 
     public enum SpawnState { Spawning, waiting, counting};
     [System.Serializable]
     public class Wave {
         public string name;
         public Transform enemy;
         public int count;
         public float rate;
     }
    
  
    public List<Wave> waves = new List<Wave>();
     private int nextWave = 0;
     public Transform[] spawnPoints;
 
     private SpawnState state = SpawnState.counting;
     public float timeBetweenWaves = 5f;
     public float waveCountdown;
 
     private float searchCountdown = 1f;
     public Transform normalEnemy;
 
     public int currentCount;
     public float currentRate;
 
     void Start() {
        waveCountdown = timeBetweenWaves;
         currentCount = 2;
         currentRate = 3;
 
     }
     void Update() {
         if (state == SpawnState.waiting) {
             if (EnemyIsAlive() == false)
             {
                 WaveCompleted();
             }
             else {
                 return;
             }
         }
 
         if (waveCountdown <= 0)
         {
             if (state != SpawnState.Spawning)
             {
                 StartCoroutine(SpawnWave(waves[nextWave]));
 
             }
         }
         else
         {
             waveCountdown -= Time.deltaTime;
         }
         
     }
     void WaveCompleted() {
     
         state = SpawnState.counting;
         waveCountdown = timeBetweenWaves;
         //TODO add more waves
       //  if (nextWave + 1 > waves.Count - 1)
        // {
             currentCount += 2;
             CreateWave(normalEnemy, currentCount, currentRate);
           
         //     Debug.Log("All Waves Complete");
         //}
        
             nextWave++;
         
     }
 
     private void CreateWave(Transform Enemy, int Count, float rate)
     {
         Wave wave = new Wave();
         currentCount = wave.count;
         currentRate = wave.rate;
         wave.name = "autoGenerated Wave";
         wave.count = Count;
         wave.rate = rate;
         wave.enemy = Enemy;
         waves.Add(wave);
      
         
 }
     bool EnemyIsAlive() {
 
         searchCountdown -= Time.deltaTime;
         if (searchCountdown <= 0f)
         {
             searchCountdown = 1f;
             if (GameObject.FindGameObjectWithTag("Enemy") == null)
             {
               
                 return false;
             }
         }
         return true;
     }
 
     IEnumerator SpawnWave(Wave _wave) {
 
         state = SpawnState.Spawning;
       //  currentCount = _wave.count;
         //currentRate = _wave.rate;
 
         for (int i = 0; i < _wave.count; i++) {
             SpawnEnemy(_wave.enemy);
             
             yield return new WaitForSeconds(1 / _wave.rate);
         }
 
         state = SpawnState.waiting;
         yield break;
     }
 
     void SpawnEnemy(Transform _enemy) {
         Transform _sp = spawnPoints[Random.Range(0, spawnPoints.Length)];
         Instantiate(_enemy, _sp.position, _sp.rotation);
     }
 }
 

why doesn't it work?