OnTriggerEnter works once, OnTriggerExit not at all?

Hi I have the player controlling a cube with a box collider and rigdidbody. When the player mesh collides with the sheep mesh I get the message "Collision Detected" but when I leave the area of the sheep mesh I do not get the message "Left Collison".

     void OnTriggerEnter(Collider other)
     {
         if(other.gameObject.tag == "sheep")
         Debug.Log ("Collision Detected");
     }
     void onTriggerExit(Collider other)
     {
         if(other.gameObject.tag == "sheep")
             Debug.Log ("Left Collison");
     }

The sheep objects have sphere colliders and sheep tags. Why is it not detecting when I have left the area of the sheep objects?