Random results with physics.OverlapSphere and AOE attacks

Hi, I am using the following code to create an area of affect magic spell.

It works fine, but seems to have some random results in hit detection. For example, I can see enemies within range, I trigger this and nothing happens, trigger it a second time and they all die.

Sometimes it triggers the first time, sometimes the third.

Can anyone look at this and find out why this seems to have seemingly random results?

             if (playerInventory.playerOneMagicPotions >= 1 && magicAttackInProgress == false)
             {
                 magicAttackInProgress = true;
                 speechPlayerScript.speech.Say("Using Spell", 3);
                 Instantiate(magicParticle, transform.position, transform.rotation);
                 Collider[] cols = Physics.OverlapSphere(transform.position, -20.0f);
                 foreach (Collider col in cols)
                 {
                     if (col.tag == "Enemy")
                     {
                         ScriptEnemyDamage e = col.gameObject.GetComponent<ScriptEnemyDamage>();
                         if (e != null)
                         {
                             e.TakeDamage(playerActualAttributes.playerOneActualMagicDamage);
                         }
                     }
                 }
                 yield return new WaitForSeconds(3.0f);
                 magicAttackInProgress = false;
             }

Any help would be greatly appreciated.