iAd does not work in the other view - objective-c

Question

I have a little problem with iAd. I used the code shown below. It works well, but only at first View, when I switch to another View, iAd does not show (although I used the exact same code). Do you know where is the problem, please? Thank you for your help.
The code that I used:
- (void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear:animated];
_adBanner = [[ADBannerView alloc] initWithFrame:CGRectMake(0, self.view.frame.size.height, 320, 50)];
_adBanner.delegate = self;
}
- (void)bannerViewDidLoadAd:(ADBannerView *)banner
{
if (!_bannerIsVisible)
{
if (_adBanner.superview == nil)
{
[self.view addSubview:_adBanner];
}
[UIView beginAnimations:#"animateAdBannerOn" context:NULL];
banner.frame = CGRectOffset(banner.frame, 0, -banner.frame.size.height);
[UIView commitAnimations];
_bannerIsVisible = YES;
}
}
- (void)bannerView:(ADBannerView *)banner didFailToReceiveAdWithError:(NSError *)error
{
NSLog(#"Failed to retrieve ad");
if (_bannerIsVisible)
{
[UIView beginAnimations:#"animateAdBannerOff" context:NULL];
banner.frame = CGRectOffset(banner.frame, 0, banner.frame.size.height);
[UIView commitAnimations];
_bannerIsVisible = NO;
}
}