Instantiating prefabs overlap regardless of Physics.CheckSphere

I've got a randomizer that selects a location on a quad and then spawns an "enemy" prefab. The problem is, no matter what I do, the prefabs inevitably overlap once or twice. Sometimes they are nearly directly on top of one another. I can't see why my use of CheckSphere is not preventing the randomizer from resetting if the location is occupied. And yes, I have swapped out the ! beforehand.

Any insight would be greatly appreciated.

 {
     public int maxEnemies = 5;
     public static int enemyCount = 0;
     public float spawnRate = 2f;
 
     public static SpawnEnemies instance = null;
     public GameObject enemy;
     public Vector2 mapPosition;
     public Vector2 randomSpawn;
 
     public float timer = 0.5f;
     public float stamp;
 
     void Start()
     {
         mapPosition = new Vector2(0f,0f);
         stamp = Time.time;
         randomSpawn = mapPosition + new Vector2(Random.Range(-5f, 5f), Random.Range(-5f, 5f));
         for (int i = 0; i < maxEnemies; i++)
         {
             MakeEnemy();
         }
             Spawn();
     }
 
     void MakeEnemy()
     {
         Instantiate(enemy, randomSpawn, Quaternion.identity);
     }
 
     public void Spawn()
     {
         if (enemyCount <= maxEnemies)
         {
             if (Time.time >= stamp)
             {
                 MakeEnemy();
                 enemyCount += 1;
                 stamp = Time.time + timer;
             }
         }
     }
 
 
     void Randomizer()
     {
         randomSpawn = mapPosition + new Vector2(Random.Range(-5f, 5f), Random.Range(-5f, 5f));
     }
 
     void Update()
     {
         Randomizer();
         while(!Physics.CheckSphere(randomSpawn,1.5f))
         {
             Randomizer();
         }
         if(enemyCount < maxEnemies)
         {
             Spawn();
         }
     }
 }