OnTriggerStay explanation?

I have created a tower defense style system, and for the most part, it works great. When objects enter the collisionbox of the turret, the turret attacks them until they. When they die, the turret stops firing.

I have noticed that there is some weirdness however, when 2 ore more enemies are in the collision box of the turret. If one dies, the turret fails to pick up on the other. And just stops firing all together.

I want to solve this with OnTriggerStay, but when I replace that function with my current OnTriggerEnter, the turret will not fire. If I just include an added OnTriggerStay function to work with my current OnTrigger Enter/Exit functions, again...the turret refuses to fire or detect enemies in it's collision box. So I'm clearly mis understanding how to use this function.

Here's my turret code;

 var rangeX : float = 0.0;
 var rangeY : float = 0.0;
 var rangeZ : float = 0.0;
 
 var projectile :         GameObject;
 var projectileSocket :     Transform; 
 var myTarget :             Transform;
 var reloadTime :        float     = 1.0;
 var nextFireTime :         float     = 0.0;
 
 
 function Update ()
 {
     if ( myTarget )
     {
         if ( Time.time >= nextFireTime )
         {
             FireProjectile ();
         } 
     } 
 }
 
 function OnTriggerEnter ( other : Collider )
 {
     if ( other.gameObject.tag == "alien" )
     {
         nextFireTime = Time.time + ( reloadTime * .5 );
         myTarget = other.gameObject.transform;
     }
 } 
 
 
 function OnTriggerExit ( other : Collider )
 {
     if ( other.gameObject.transform == myTarget )
     {
         myTarget = null;
     }
 }
 
 function FireProjectile ()
 {
     audio.Play ();
     nextFireTime = Time.time + reloadTime;    
     Instantiate(projectile, projectileSocket.position, projectileSocket.rotation);
 }
 
 function OnDrawGizmos ()
 {
     Gizmos.color = Color.white;
     Gizmos.DrawWireCube ( transform.position, Vector3 ( rangeX, rangeY, rangeZ ) );
 }