C# cool down not looping

I've made a 5 second cool down function starting upon button click. It doesn't loop though, only works once. What's wrong?

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class button1 : MonoBehaviour {
 
     public bool startTimer = false;
     public bool ready = true;
     int counter = 0;
     public int coolDown = 5;
     public float Counter = 0f;
     public Image buttonImage;
 
     // Update is called once per frame
     public void Clicked()
     { 
         if (ready == true) {
             counter = counter + 1;
             Debug.Log ("Button1: Clicked");
             buttonImage.fillAmount -= coolDown * Time.time;
             ready = false;
             startTimer = true;
         }
     }
         void Update ()
         {
         if (startTimer == true) 
         {
             Counter = Time.time;
         }
         if (Counter >= coolDown) 
         {
             ready = true;
             startTimer = false;
             Counter = 0f;
             buttonImage.fillAmount += coolDown * Time.time;
         }
         }
 }