OnCollisionEnter suddenly stopped working

This is by far the weirdest thing. I implemented an OnCollisionEnter hit detection with my enemy and it worked great at first in the editor, as soon as I built and ran the game it stopped working in not only the build but the editor as well!

My enemy script has this:

 function OnCollisionEnter (Col : Collision) {
     if (Col.gameObject.tag == "HitBox0") {
         var player = GameObject.FindWithTag("Player"); //Find the player
         player.GetComponent(PlayerControl).OnAttack();
         print ("Hit detected!");
     }
 }
 
 function Hit () {
     CurEnemyHealth -= knfdmg;
 }
 

My player has this which initiates the "hit" when PlayerIsAttacking = true, in which when I attack it sets that variable to true.

 //Attacking enemy
 function OnAttack () {
     if (PlayerIsAttacking == true || KnifeAttkFwd == true || KnifeAttkUp == true || KnifeAttkDown == true) {
         var enemy = GameObject.FindWithTag("Enemy"); //Find the enemy
         enemy.GetComponent(ZombieAI).Hit();
     }
 }

I don't get why it worked perfectly the first time then just decides to stop working after I built the game?

The HitBox object has a mesh collider and a rigidbody. All of the tags and scripts are set to their respective objects so what gives?