这就是Google所说的

This is what google says

可能是您刚刚创建了一个新的广告单元ID，而
则请求了实时广告。广告可能需要几个小时才能开始
的获取如果是这种情况，则可以投放广告。如果您正在接收测试广告
，那么您的实施情况还不错。请稍等几个小时，看看您
是否能够接收实时广告。如果没有，可以发送给我们您的广告单元ID
，供我们查看。

"It could be that you have only recently created a new Ad Unit ID and requesting for live ads. It could take a few hours for ads to start getting served if that is that case. If you are receiving test ads then your implementation is fine. Just wait a few hours and see if you are able to receive live ads then. If not, can send us your Ad Unit ID for us to look into."

因此，基本上，您必须等待几个小时：)

so basically you have to wait for a few hours :)

https://groups.google.com/forum/#!category-topic/google-admob-ads-sdk/android/fBe3YL3ffpo