UIViewController only viewWillAppear fires

Question

I have a UIViewController as the root view controller of a UINavigationController. And when it appears for the first time, or it becomes visible as a result of the user pressing back viewWillAppear fires but viewDidAppear does not.

Also when the next view is pushed to be visible and the root dissapears, neither of the disappear methods fire.

This is on the iPad and the UINavigationController is not fullscreen.

However on the iPhone, on the DrillDownSave Apple sample code all 4 viewDid/WillAppear/Dissapear methods fire on that.

So why does it work in the Apple sample code but not in mine, the only difference I can see is that I create the UINavigationController programmatically, whereas in the Apple code it comes from a nib.