Health bar UI not working after building.

I have created a health bar that works in editor, but once I build my game, it just does not decrease when I take damage. (I can take damage, but the health bar is not showing it.)
(Pictures of inspector of health bar and hierarchy attached.)

Here is the health bar script:

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using UnityEngine.UI;
5.  
6. public class HealthBarHUD : MonoBehaviour {
7.  
8.     public Image full;
9.     public Image mid;
10.     public PlayerHealthSystem phs;
11.  
12.     public float midPercent = 0.5f;
13.     public float dangerPercent = 0.15f;
14.     public float lowPercent = 0.3f;
15.  
16.     public PlayerHealth player;
17.     // Use this for initialization
18.     void Start () {
19.  
20.         player = GameObject.FindGameObjectWithTag("Play").GetComponent<PlayerHealth>();
21.  
22.         if ((phs.health/player.p_max_health) < this.midPercent)
23.         {
24.             full.fillAmount = 0;
25.             mid.fillAmount = (2*phs.health ) / player.p_max_health;
26.         }
27.         else
28.         {
29.             mid.fillAmount = 1;
30.             full.fillAmount = (phs.health / player.p_max_health);
31.         }
32.        
33.     }
34.    
35.     // Update is called once per frame
36.     void Update () {
37.         if ((phs.health / player.p_max_health) < this.midPercent)
38.         {
39.             mid.fillAmount = (2*phs.health) / player.p_max_health;
40.             full.fillAmount = 0;
41.         }
42.         else
43.         {
44.             mid.fillAmount = 1;
45.             full.fillAmount = (phs.health / player.p_max_health);
46.         }
47.     }
48. }
49.  

HealthSystem (To convert current health into "health" variable):

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class PlayerHealthSystem : MonoBehaviour {
6.     public float health;
7.     public PlayerHealth player;
8.  
9.     // Use this for initialization
10.     void Start () {
11.         player = GameObject.FindGameObjectWithTag("Play").GetComponent<PlayerHealth>();
12.     }
13.    
14.     // Update is called once per frame
15.     void Update ()
16.     {
17.         health = player.p_curHealth;
18.     }
19. }
20.  

Is this caused by a coding error, or other reasons? Thanks.