you need to inherit your window or any viewmodel class from INotifyPropertyChanged
otherwise it will not Notifies clients that a property value has changed.
for your case it could be like
public partial class Window1 : INotifyPropertyChanged
{
// Class code goes here;
public event PropertyChangedEventHandler PropertyChanged;
protected void RaisePropertyChanged(String propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}