IPhone : iOS: UIBackgroundFetch - PerformFetch not called

I want to update my app in background, so I added Required background modes "fetch" to my info.plist. In my FinishedLaunching I added:

UIApplication.SharedApplication.SetMinimumBackgroundFetchInterval (UIApplication.BackgroundFetchIntervalMinimum);

And overwriting the method PerformFetch:

public override void PerformFetch (UIApplication application, Action<UIBackgroundFetchResult> completionHandler)
{
    Console.WriteLine ("PerformFetch called...");

//Return no new data by default
var result = UIBackgroundFetchResult.NoData;
...

completionHandler (result);
}

But the method never get called... :(

What's the problem, I can't find any mistakes? Thank you for help.