Why does my GUI not refresh although I have implemented Binding (MVVM)

Hi Guys,

So the point of complete desperation has arrived... I work with Xamarin.Forms and MVVM. I created a Content page containing a StackLayout. Then I added a Child to the StackLayout wich contains a ScrollView. Here is the code of the child:

namespace VCP_Touch
{
    public partial class Ruhestand : ScrollView
    {
        public Ruhestand ()
        {
                InitializeComponent ();
        }
    }
}

and the Xaml:

<?xml version="1.0" encoding="UTF-8"?>
<ScrollView xmlns="http://xamarin.com/schemas/2014/forms" 
            xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
            xmlns:local="clr-namespace:VCP_Touch;assembly=VCP_Touch"
            x:Class="VCP_Touch.Ruhestand"
            Orientation="Horizontal">
    <ScrollView.BindingContext>
        <local:RuhestandModel />
    </ScrollView.BindingContext>
    <StackLayout>
        <Label  Text="{Binding Eingabe}"
                IsVisible="true"/>

        <Button Text="Refresh"
                Command="{Binding Refresh}"
                HorizontalOptions="FillAndExpand"/>
    </StackLayout>
</ScrollView>

I added a ViewModel, which is named "RuhestandModel":

namespace VCP_Touch
{
    public class RuhestandModel : INotifyPropertyChanged
    {
        public RuhestandModel ()
        {
            Eingabe = "Default";

            this.Refresh = new Command ((nothing) => {
                VO_Datenerfassung test2 = new VO_Datenerfassung ();
                this.Eingabe = test2.Interessent.Vorname;
            });
        }
        string _Eingabe;

        public string Eingabe {
            get {
                return _Eingabe;
            }
            set {
                if (_Eingabe != value) {
                    _Eingabe = value;
                    OnPropertyChangedModel ("Vorname");
                }
            }
        }
        public ICommand Refresh { protected set; get; }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChangedModel(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, 
                    new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}

"VO_Datenerfassung" is a class, containing several Properties. It can be changed with another button (and it works!!)...The code is not displayed here.
Coming to my problem. Principly the code works fine! When I click the "Refresh" button, the Command Refresh in the constructor "public RuhestandModel ()" is activated. Then also the field "_Eingabe" is changed. But on the GUI the text of the Label (which is bound to Eingabe) does not change.
Where is my fault? I looked at all the examples on the Xamarin Developpers page and I searched the Forum, but I did not found any answer.
You see, I'm truly desperate, because the Binding thing HAS to work, otherwise App programming is not possible...
Thanks in advance!

Ben