OnCollisionEnter Not Working After Game Build

This code works great in the editor, but after I build the game (I’ve tried building both standalone and web applications) the OnCollisionEnter function no longer runs. If it works correctly, when the player runs into a monster the game ends. Here’s the code:

void OnCollisionEnter(Collision collision)
   {
      if (this.tag == "Player" && collision.collider.tag == "Monster")
      {
	game.gameOver ();
      }
}

Any ideas on why this wouldn’t work after the build?

@nodarbisc, maybe use a OnTriggerEnter() for the monster as well see if that makes a difference. I am confused, because if it works in the editor then it should work when the game is built.