Enemy AI Unable to See In Varying Heights?

I have an empty attached to the enemy that constantly turns to face you and casts raycasts to determine if you're in the enemy's sight and not behind a wall. It works just fine if the player is around the same height level as the enemy but, if you're below them even a slight amount, they seem to act as though they can't see you at all. Here's the script:

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class SightSpot : MonoBehaviour
6. {
7.  
8.     GameObject Player;
9.  
10.     void Start ()
11.     {
12.         Player = GameObject.Find("MainCharacter");
13.     }
14.  
15.     void Update()
16.     {
17.         if (Player != null)
18.         {
19.             Vector3 targetPostition = new Vector3(Player.transform.position.x,
20.                                            this.transform.position.y,
21.                                            Player.transform.position.z);
22.             this.transform.LookAt(targetPostition);
23.         }
24.     }
25. }
26.  

I know this is due to the y axis being excluded but, for some reason, setting the targetPosition's transform.position.y to Player causes the enemy to become completely blind. Is there a way around this?