Problem with OnTriggerEnter()

Hey guys! I'm making a Doom-style FPS (meaning the Y-axis is locked). Anyways I want the raycasts from shooting to aim up or down based on the enemy the player is looking at. The best workaround i could figure out for this is to attach a trigger box collider to an empty object on the camera that determines the relative Y angle to the enemy when AI enters the box. I'm having trouble getting the OnTriggerEnter() to call (whereas OnTriggerExit() works just fine). Here's a visual example:

• There is a collider and rigidbody attached to ALL AI

• The trigger box attached to the camera is large enough to completely fit the AI

• The script is attached to the same game object as the trigger

• They're all on the same layer (which is set to respect physics on collision with itself)

Here's some of the script:

 private var colliderCount = 0;
 
 function OnTriggerEnter(hit : Collider){
     Debug.Log(colliderCount + "registered");
     if (hit.transform.tag == "AI") {
     colliderCount++;
     Debug.Log(colliderCount);
     }
 }
 
 function OnTriggerExit(hit : Collider){
     if (hit.transform.tag == "AI") {
     colliderCount--;
     Debug.Log(colliderCount + "registered");
     }
 }

I'd be really happy if someone could help me figure it out (or provide a better workaround than the one I'm using), and I'd be happy to provide pictures of my kitten as a 'thank you'!