Android AdMob Mediation - Not Receiving Ads With Mediated Networks

I am trying to set up an Android app to serve ads from mediated networks (which, by the way, has no explanation anywhere else) and this is what I have done so far:

1) I copied the publisher id to the xml view that will show ads and now only AdMob ads are displayed on the device.

2) I created ad slots on linked networks and linked them to my AdMob reseller using identifiers provided by the networks.

3) I have imported the required SDKs and network adapters in my project

4) I added the required permissions and network actions in my manifest

Now this is my code:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    FlurryAdapterExtras flurryAdapterExtras = new FlurryAdapterExtras();
    flurryAdapterExtras.setLogEnabled(true);
    FlurryAgent.setLogLevel(Log.VERBOSE);

    MillennialAdapterExtras millenialAdapterExtras = new MillennialAdapterExtras();

    AdView mAdView = (AdView) findViewById(R.id.adView);
    AdRequest adRequest = new AdRequest.Builder()
        .addTestDevice("600447FDC2D619692F94F848E532BAC3")
        .addNetworkExtras(flurryAdapterExtras)
        .addNetworkExtras(millenialAdapterExtras)
        .build();
    mAdView.loadAd(adRequest);

}

      

        
        
        
      

    

And I only get ads from AdMob, there is not even any mention of other networks in LogCat ...

What could be wrong?