Admob 的回调不是每次都在 Swift 中触发

问：

我使用 SwiftUI 框架和 SwiftUI App 生命周期在 Swift 中制作了一个应用程序。

我使用该类来管理插页式广告：

class Interstitial : NSObject, GADFullScreenContentDelegate, UIApplicationDelegate{
    var interstitial : GADInterstitialAd?
    
    let adUnitID: String = "ca-app-pub-3940256099942544/4411468910"
    
    override init() {
        super.init()
        loadInterstitial()
    }
    
    func loadInterstitial(){
        let request = GADRequest()
        GADInterstitialAd.load(withAdUnitID: adUnitID, request: request) { [self] (ad, err) in
            if(err !=  nil){
                print(String(describing:err?.localizedDescription))
            }
            else {
                interstitial = ad
                interstitial?.fullScreenContentDelegate = self
            }
        }
    }
    
    func showAd(){
        let root = UIApplication.shared.windows.first?.rootViewController
        if interstitial != nil {
            interstitial!.present(fromRootViewController: root!)
        }
        else {
            nc.post(name: Notification.Name("UserClosedAd"), object: nil)
        }
    }
    
    func adDidDismissFullScreenContent(_ ad: GADFullScreenPresentingAd) {
        print("ad closed")
        nc.post(name: Notification.Name("UserClosedAd"), object: nil)
        return
    }
    func ad(_ ad: GADFullScreenPresentingAd, didFailToPresentFullScreenContentWithError error: Error) {
        print(error.localizedDescription)
        print("ad failed to be presented")
        nc.post(name: Notification.Name("UserClosedAd"), object: nil)
        return
    }
    func adDidPresentFullScreenContent(_ ad: GADFullScreenPresentingAd) {
        print("ad presented")
        return
    }
}

我以这种方式展示广告：

let pub = NotificationCenter.default
        .publisher(for: NSNotification.Name("UserClosedAd"))
var interstitial : Interstitial = Interstitial()

var body: some View {
    Button(action:
            {
                self.interstitial.showAd()
            }) {
        Image(systemName: "text.bubble")
            .foregroundColor(.red)
        Text("Show ad")
    }
    .font(.system(size: 30, weight: .semibold, design: .rounded))
    .padding(30)
    .onAppear(perform: {
        interstitial.loadInterstitial()
    })
    .onReceive(pub, perform: { _ in
        newView = true
        print("Ad closed")
    })
}

问题是有时回调有效，但有时无效。当我在未连接到 Xcode 的真实设备或模拟器上使用该应用程序时，回调永远不会起作用。

可能是什么问题？

提前致谢， 翁文