Thanks to all,
finally what happen is as follows.
The above behaviour is not what i assume it is. When a new view is loaded the button is disappear by default and only if i move my finger here & there on the location button used to be it appears. Further when i keep the view still, after it loaded, about 15 seconds, button appears.
So i suspect this is with "viewDidLoad" method. (Code related to this UINavigationBar is in viewDidLoad method.
When I move above code to "viewDidAppear" method it start to work again.