Ios 在间隙Ad请求失败后再次加载Ad

我想在特定的时间显示一个插播广告。我先把广告装进去，然后再展示。如果广告无法加载，我会用定时器从DIDFILETERECIEVEAD再次加载。但现在的问题是，didFailToRecieveAd被称为multipleTimes

检查下面的代码

- (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];
}

原因可能是什么