View not updating after a property in ViewModel is updated - xaml

Question

So I'm trying to implement a ListBoxItem that has a TextBlock that binds to a property in the viewmodel. But for some reason the view isn't being updated even after the viewmodel property is updated. Can anyone give me some pointers?
In the view (xaml)
<phone:Pivot HorizontalAlignment="Left" Height="748" Margin="10,10,0,0" Grid.Row="1" Title="pivot" VerticalAlignment="Top" Width="460">
<StackPanel x:Name="POIStackPanel" DataContext="{StaticResource POIViewModel}">
<ListBoxItem x:Name="SelectedPOIItem">
<TextBlock Text="{Binding SelectedPickupPOI.label}" />
</ListBoxItem>
phone:PivotItem x:Name="GooglePivot" Header="Google">
<ListBox x:Name="GooglePOIList"
ItemsSource="{Binding GooglePOICollection}"
HorizontalAlignment="Left"
Height="441"
Width="436"
SelectedItem="{Binding SelectedPickupPOI, Mode=TwoWay}"
ItemTemplate="{StaticResource POIItemTemplate}" />
</phone:PivotItem>
</StackPanel>
</phone:Pivot>
In the viewmodel
public ObservableCollection<PointOfInterest> GooglePOICollection { get; private set; }
public PointOfInterest SelectedPickupPOI
{
get
{
return _selectedPickupPOI;
}
set
{
_selectedPickupPOI = value;
NotifyPropertyChanged("SelectedPickupPOI");
}
}
public PointOfInterestViewModel()
{
this.GooglePOICollection = = new ObservableCollection<PointOfInterest>();
this.SelectedPickupPOI = new PointOfInterest();
}
public event PropertyChangedEventHandler PropertyChanged;
protected void NotifyPropertyChanged(String propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (null != handler)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
My PointOfInterest Models
public abstract class PointOfInterest : INotifyPropertyChanged
{
public abstract string id { get; set; }
public abstract string label { get; set; }
}
public class GooglePOI : PointOfInterest
{
private string _label;
public override string label
{
get { return _label; }
set
{
_label = value;
NotifyPropertyChanged("label");
}
}
}
Update
so I found out that if I do _selectedPickupPOI.label = value.label; it will update the view correctly. What am I doing wrong?