Android : Admob: no "impression" (android)

I' ve followed all the necessary steps for adding banner in my android application with Admob.

I 've registered my device as test device and banner is displayed as below :

However, 5 days after publication on Google Play, I got this :

Does someone know why impression rate is so low ?

My android code :

 // CREATING a Smart Banner View
 adView = new AdView(this);
 adView.setAdUnitId("ca-app-pub-xxxxxxxxxxx");
 adView.setAdSize(AdSize.FULL_BANNER);
 adView.refreshDrawableState();

 AdRequest adRequest = new  AdRequest.Builder().addTestDevice("xxxxxxxxx").build();
 adView.loadAd(adRequest);

 frameLayout.addView(adView, adViewLayoutParams);

Thank you