android - CardView 内的 NativeExpressAdView

Question

如果我从 CardView 切换到 Framelayout 广告在 Cardview 中显示时不显示广告出现任何原因？

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:card_view="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@color/banner_bg"
android:orientation="vertical"
card_view:cardElevation="@dimen/card_elevation_height">


<com.google.android.gms.ads.NativeExpressAdView xmlns:ads="http://schemas.android.com/apk/res-auto"
    android:id="@+id/adView"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    ads:adSize="FULL_WIDTHx148"
    ads:adUnitId="@string/test_unit_id_m" />