Struggling with integrating iAd

Question

I have a tab bar application which have table views as sub views. I need to show ad banner at bottom just above the tab bar. The ad banner sometimes is seen and sometimes not for a long time. Sometimes it leaves the white view when as not available and sometimes not. I am not getting what is happening here:

I am using:

- (void)viewDidLoad
{
    [super viewDidLoad];

    CGRect frame=CGRectZero;
    frame.size = [ADBannerView sizeFromBannerContentSizeIdentifier:ADBannerContentSizeIdentifierPortrait];
    // Place frame at the bottom edge of the screen out of sight
    frame.origin = CGPointMake(0.0, 317);

    // Now to create and configure the banner view
    ADBannerView *adView = [[ADBannerView alloc] initWithFrame:frame];
    adView.requiredContentSizeIdentifiers = [NSSet setWithObject:ADBannerContentSizeIdentifierPortrait];
    // Set the delegate to self, so that we are notified of ad responses
    adView.delegate = self;
    adView.hidden = YES;
    [self.view addSubview: adView];
}



- (void)bannerViewDidLoadAd:(ADBannerView *)banner
{
    NSLog(@"ad now available");

    banner.hidden = NO;
    // Get a brand new frame
    CGRect newFrame=CGRectZero;
    CGPoint frameOrigin=CGPointZero;
    // Set the origin
    frameOrigin=CGPointMake(0.0, CGRectGetMaxY(self.view.bounds));
    newFrame.origin=frameOrigin;

    // Set the size
    newFrame.size=[ADBannerView sizeFromBannerContentSizeIdentifier:ADBannerContentSizeIdentifierPortrait];

    CGFloat bannerHeight = newFrame.size.height;
    CGFloat bannerOffset=0.0;

    // Determine where the new frame should be
    if (!self.isBannerVisible)
    {
        // It should be visible, raise it up
        bannerOffset=-bannerHeight;
    }

    CGRect offSetRect=CGRectOffset(newFrame,0.0f,bannerOffset);
    [UIView animateWithDuration:0.2
                     animations:^{banner.frame= offSetRect;}
                     completion:^(BOOL finished){
                         if (bannerOffset<0){
                             self.isBannerVisible=YES;
                         }else{
                             self.isBannerVisible=NO;
                         }
                     }
     ];

}



- (void)bannerView:(ADBannerView *)banner didFailToReceiveAdWithError:(NSError *)error
{

     NSLog(@"error is===>%@",error);
}

- (BOOL)bannerViewActionShouldBegin:(ADBannerView *)banner willLeaveApplication:(BOOL)willLeave
{
    return YES;
}

Sometimes error is:

Error Domain=ADErrorDomain Code=0 "The operation couldn’t be completed. Unknown error" UserInfo=0x8c53410 {ADInternalErrorCode=0, ADInternalErrorDomain=ADErrorDomain, NSLocalizedFailureReason=Unknown error}

sometimes it is:

Error Domain=ADErrorDomain Code=3 "The operation couldn’t be completed. Ad inventory unavailable" UserInfo=0x7e64780 {ADInternalErrorCode=3, ADInternalErrorDomain=ADErrorDomain, NSLocalizedFailureReason=Ad inventory unavailable}

Please someone help!!