AdMob not showing ads on Android.. Getting "network error" in logcat

Hi everyone,

I'm programming my first game with unity, and I've been working on for a month. I wanted to use Admob from google to display ads but i can't make it work on my device (tried on Nexus 5 and S2).

I followed the tutorial from google : https://github.com/googleads/googleads-mobile-plugins/tree/master/unity

I manage to make the GoogleAdsDemoScript work on Unity, i.e it displays "Dummy CreateBannerView" and "Dummy ShowBannerView", which means it should work fine. But I compiled it on my device and ads don't show.. I 've tried to debug it thanks to adb logcat, and i got this error everytime I clicked on a button :

I/Unity ( 9078): HandleFailedToReceiveAd event received with message: Network Error

I/Unity ( 9078):

I/Unity ( 9078): (Filename: ./artifacts/generated/common/runtime/UnityEngineDebug.gen.cpp Line: 56)

I searched on the net, but no answer found :/

Here is my AndroidManifest.xml :

 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="com.google.unity"
     android:versionName="1.0"
     android:versionCode="1">
   <uses-sdk android:minSdkVersion="9"
       android:targetSdkVersion="19" />
   <!-- Google Mobile Ads Permissions -->
   <uses-permission android:name="android.permission.INTERNET"/>
   <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
   <application>
     <!-- Denote the referenced Google Play services version -->
     <meta-data android:name="com.google.android.gms.version"
         android:value="@integer/google_play_services_version" />
     <!-- Google Mobile Ads Activity -->
     <activity android:name="com.google.android.gms.ads.AdActivity"
         android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize"/>
   </application>
 </manifest>

Thanks for any help !