Instatiante object's script is not working in run time

 using UnityEngine;
 
 public class EnemyManager : MonoBehaviour
 {
     public PlayerHealth playerHealth;
     public GameObject enemy;
     public float spawnTime = 3f;
     public Transform[] spawnPoints;
 
     public static bool once;
 
     void Start ()
     {        
         Invoke("Spawn", spawnTime);             
     }
 
     void Update()
     {        
         if (once)  
         {            
             Invoke("Spawn", spawnTime);
             once = false;        
         }  
     }
 
 
     void Spawn ()
     {
         if(playerHealth.currentHealth <= 0f)
         {
             return;
         }
         int spawnPointIndex = Random.Range (0, spawnPoints.Length);
         Instantiate (enemy, spawnPoints[spawnPointIndex].position, spawnPoints[spawnPointIndex].rotation);
     }
 }
 

In runtime, first enemy is instantiated in Start function. For the first enemy, all scripts attached to enemy are working well. However, scripts attached to enemy which are instantiated in Update function are not working. If I remove "once" bool variable and if statement, scripts attached to enemy are working well. Problem is related to bool variable and if statement. Why?