Ads not working on some devices

Hello guys,

Unity ads work on P30 Lite and A20 2018 but I get no ads on J5 2018. Absolutely no signs of it working. On the other phones it works perfectly. I have reviewed my project settings, the store id, everything is fine. What could be going on?

Code (CSharp):

1. using UnityEngine;
2. using UnityEngine.Advertisements;
3.  
4. public class AdManager : MonoBehaviour {
5.  
6.     string gameId = "xxxxxxx";
7.     string myPlacementId = "video";
8.     int delay = 0;
9.  
10.     void Start () {
11.         Advertisement.Initialize (gameId, false);
12.     }
13.  
14.     public void ShowAd(){
15.         if(delay == 3){
16.             if(Advertisement.IsReady(myPlacementId)){
17.                 Advertisement.Show(myPlacementId);
18.                 delay = 0;
19.         }
20.         }else{
21.             delay++;
22.         }
23.     }
24. }