共享的iAd Banner不会出现在TableView之间

Question

根据http://www.youtube.com/watch?v=u5XcVnHCQ0w的教程，我实现了共享的iAd横幅视图，以显示两个表的标头位置。两个表控制器位于导航控制器下。横幅视图显示在第一个表中，但未显示在下一个表中。

我在UItableViewControllers中实现的相同代码

     - (AppDelegate *) appdelegate {
        return (AppDelegate *)[[UIApplication sharedApplication] delegate];
    }

     -(CGFloat)tableView:(UITableView *)tableView heightForHeaderInSection:(NSInteger)section{
      return 100.0;
    }

    -(void) viewWillAppear:(BOOL)animated{
        _UIiAD = [[self appdelegate] UIiAD];
        _UIiAD.delegate = self;

        [_UIiAD setFrame:CGRectMake(0,21,320,50)];
        [self.view addSubview:_UIiAD];
    }

    -(void) viewWillDisappear:(BOOL)animated{
        _UIiAD.delegate = nil;
        _UIiAD=nil;
        [_UIiAD removeFromSuperview];
    }

    -(void)bannerViewDidLoadAd:(ADBannerView *)banner{
        NSLog(@"ads loaded");
        [UIView beginAnimations:nil context:nil];
        [UIView setAnimationDuration:1];
        [_UIiAD setAlpha:1];
        [UIView commitAnimations];
    }

    -(void)bannerView:(ADBannerView *)banner didFailToReceiveAdWithError:(NSError *)error{
        NSLog(@"ads not loaded");
        [UIView beginAnimations:nil context:nil];
        [UIView setAnimationDuration:1];
        [_UIiAD setAlpha:0];
        [UIView commitAnimations];
    }