My health bar is not filling to the health variable please help!

//start

using UnityEngine; 
using System.Collections;
using UnityEngine.UI;

public class EnemyHealth : MonoBehaviour { 
	public int maxHealth = 500; 
	public int curHealth = 500;
	public Image Healthbar;

	//initialization 
	void Start ()
	{ 
	  Healthbar = transform.FindChild ("EnemyCanvas").FindChild ("HealthBG").FindChild ("Health").GetComponent<Image> ();
	} 

	// Update 
	void Update () { 
		if (curHealth <1){ 
			Destroy(gameObject);
			Healthbar.fillAmount = (float)curHealth / (float)maxHealth;			 
		} 
	}
	void OnCollisionEnter(Collision col) { 
		if (col.gameObject.tag == "BubbleBullet"){ 
			curHealth -= 20;
			Destroy(col.other);
		} 
	} 
}

Please help me!

if (curHealth <1){
Destroy(gameObject);
Healthbar.fillAmount = (float)curHealth / (float)maxHealth;
}

The fill amount is only calculated (updated) if the current health is below 1. You probably want to move the calculation outside the if statement (?).