Button waits 3 clicks before working

Hi,

I have quite a problem here: I created a button "Menu" thats works like a dropdown. Basically, it makes a panel with icons appear/disappear (with Gameobject.SetActive()).

Here is the script for the dropdown menu. I attached it to the event OnClick().

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class Drop_Down_Menu : MonoBehaviour
 {
     public GameObject panel;
 
     private bool isActive;
 
     public void Start ()
     {
         isActive = false;
         panel.SetActive (false);
     }
 
     public void DropDown (GameObject panel)
     {
         if (isActive) {
             panel.SetActive (false);
             isActive = false;
         } else if (!isActive) {
             panel.SetActive (true);
             isActive = true;
         }
 
     }
 }

It seems to work quite well, except that I have to wait the 3rd click for it to activate and actually make my panel visible. I'm sure I've seen a topic like this in the forum, but I can't find it anymore.

Did anyone have the same problem before?

Thanks for your help!