OnElementPropertyChanged only fires once

I have this bindable property for my custom renderer , when I change the property for the first time
OnElementPropertyChanged in custom renderer gets fired but on next change it doesn't fire.
any idea about why it works only once?

public static readonly BindableProperty NumberProperty =
            BindableProperty.Create<MyView, string>(p => p.Number, null,BindingMode.TwoWay);

public string  Number { get { return (string)GetValue(NumberProperty ); } 
            set { SetValue (NumberProperty, value); } } // it would rise ElementPropertyChanged event}

and in my custom renderer

protected override void  OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            // handle elemnt property change
        }