Facebook的NativeAdsManager总是提供相同的广告

Question

我正在按预期使用适用于Android的Facebook NativeAdManager：

nativeAdsManager = new NativeAdsManager(context, "my_placement_id", 3);

应该为我加载3个不同的广告，它们使我可以使用以下方法将其放入适配器：

NativeAd adItem = nativeAdsManager.nextNativeAd();

但是由于某些原因，我总是得到相同的广告。根据他们的参考：

The NativeAdsManager provides a mechanism to fetch a set of NativeAds and then use them within your application. The recommended usage is to call nextNativeAd() at the moment when you are about to render an ad. The NativeAdsManager can hold at most 10 unique ads at once but supports giving out as many ads as needed by cloning over the set of NativeAds it got back from the server which can be useful for feed scenarios.

我在这里想念东西吗？