Android : Getting Ad request successful, but no ad returned due to lack of ad inventory error for loading DFP ads in Nexus 7 device

I am unable to load DFP ads in Nexus 7 tablet device. I am getting failed message Ad request successful, but no ad returned due to lack of ad inventory.

Using SMART_BANNER adsize I have tested on some smart phones(Google nexus,nexus 5,s3,s4) and Samsung 10 tablet, it works perfectly in all devices but in Nexus 7 ad failed to load due to the lack of ad inventory.

My xml code snippet

<com.google.ads.doubleclick.DfpAdView
    xmlns:ads="http://schemas.android.com/apk/lib/com.google.ads"
    android:id="@+id/adView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    ads:adUnitId="MY_AD_UNIT_ID"
    ads:adSize="SMART_BANNER"
    ads:loadAdOnCreate="true"/>

Please suggest me to fix this issue ASAP.