INotifyPropertyChanged Events are not firing in Xamarin.Forms.Labs.Mvvm.ViewModel Extension

Hi,
In our sample application, we have an Auto complete control and two picker controls (Data binding as "List Collection" by calling WCF REST Service).
We have used "OnPropertyChanged" Event to select the Picker SelectedIndex ,Enable/Disable the control. But INotifyPropertyChanged events are not firing.

Ex: we used as "public class SearchRequestFilterViewModel : Xamarin.Forms.Labs.Mvvm.ViewModel" it doesn't call the OnPropertyChanged event.
We have tried as "public class SearchRequestFilterViewModel : Xamarin.Forms.Labs.Mvvm.ViewModel,INotifyPropertyChanged" but the result remains same.

Kindly suggest how to use INotifyPropertyChanged with Xamarin.Forms.Labs.Mvvm.ViewModel class extension ?