react-native-admob: AdMobBanner is not displayed

I have tried to add admob to my project . my project is running correctely without error but no banner is dispayed . my code is : <ScrollView> <AdMobBanner bannerSize="banner" adUnitID="ca-app-pub-3207834634154483/6974060636" testDeviceID="EMULATOR" didFailToReceiveAdWithError={this.bannerError} adViewDidReceiveAd ={null} adViewDidDismissScreen={null} adViewWillLeaveApplication={null}/> </ScrollView>

Can someone tell me what are probable reason why my banner is not dispayed .

Thanks