在插页式广告请求失败后再次加载广告

Question

我想在特定时间显示插页式广告。我手头加载广告然后再显示。如果Ad无法加载，我会使用计时器从didFailToRecieveAd再次加载它。但现在问题是 didFailToRecieveAd被称为multipleTimes 。

检查以下代码

- (id) initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil{
    if ((self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil])) {
        // Custom initialization
    }
    interstitial_ = [[GADInterstitial alloc] init];
    interstitial_.adUnitID = "XXX";
    interstitial_.delegate = self;
    [[GADRequest request] setTestDevices:[NSArray arrayWithObjects:@"75effd1496dd0f332738f26d48fcff01", nil]];
     [self loadAd];
    return self;
}
-(void) loadAdAgain{
    [interstitial_ dealloc];
    interstitial_ = [[GADInterstitial alloc] init];
    interstitial_.adUnitID = [[PreferenceHelper sharedHelper] getString:@"ad_unit_full_screen"];
    interstitial_.delegate = self;
    [[GADRequest request] setTestDevices:[NSArray arrayWithObjects:@"75effd1496dd0f332738f26d48fcff01", nil]];
    loaded = NO;
    [self loadAd];
}

-(void) loadAd{

    [interstitial_ loadRequest:[GADRequest request]];
}

-(void) interstitial:(GADInterstitial *)ad didFailToReceiveAdWithError:(GADRequestError *)error{
    [self trackEvent:@"ad_ios" action:@"Failed to load" label:[NSString stringWithFormat:@"ads_%d",adPresentedCount] value:[[PreferenceHelper sharedHelper] getInt:@"games"]];
    [NSTimer scheduledTimerWithTimeInterval:5.0f target:self selector:@selector(loadAdAsync) userInfo:nil repeats:NO];
}

可能是什么原因？