Saved data lost after kill app using NSUserDefaults

I use NSUserDefaults in my setting. App works well even I press home key let app into background, but if I kill the app, the data save in NSUserDefaults will lost. Here is my code. I have use synchronize. The First initialize:

 if (![userDefaults integerForKey:
          kORFootageAirPlayModeKey])
    {
        [userDefaults setInteger:TRUE forKey:kORFootageAirPlayModeKey];
    }
    [userDefaults synchronize];

Read value out in a viewController:

airPlayMode = [[NSUserDefaults standardUserDefaults]integerForKey:kORFootageAirPlayModeKey];

Set it in a action:

- (IBAction)changeAirPlayStatus:(id)sender
{

    if (sender)
    {
        airPlayMode = [sender tag];

        [[NSUserDefaults standardUserDefaults] setInteger:airPlayMode forKey:kORFootageAirPlayModeKey];
        [[NSUserDefaults standardUserDefaults] synchronize];
    }
.....

}