Android Studio中AdMob广告无法加载（ErrorCode:0）求助

我在给Android应用集成AdMob广告时遇到了棘手的问题——持续收到报错信息：

There was a problem getting an ad response. ErrorCode: 0
Failed to load ad: 0

哪怕我直接照搬了官方教程里的代码，还是出现一模一样的错误，实在摸不着头绪。以下是我当前使用的代码：

布局文件（activity_main.xml） #

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="michaelsinn.contact.gmail.com.testads.MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <com.google.android.gms.ads.AdView xmlns:ads="http://schemas.android.com/apk/res-auto"
        android:id="@+id/adView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_alignParentBottom="true"
        ads:adSize="SMART_BANNER"
        ads:adUnitId="ca-app-pub-3940256099942544/6300978111">
    </com.google.android.gms.ads.AdView>

</android.support.constraint.ConstraintLayout>

Activity代码（MainActivity.java） #

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.AdView;
import com.google.android.gms.ads.MobileAds;

public class MainActivity extends AppCompatActivity {
    private AdView mAdView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        MobileAds.initialize(this,"ca-app-pub-3940256099942544~3347511713");
        mAdView = findViewById(R.id.adView);
        AdRequest adRequest = new AdRequest.Builder().addTestDevice("[Correct test device id is here]").build();
        mAdView.loadAd(adRequest);
    }
}

有没有大佬能帮我排查下问题？万分感谢！

内容的提问来源于stack exchange，提问作者Michael Sinn