IsChecked Binding not working in MenuItem in a ContextMenu

I have an MVVM Application and want to add a ContextMenu.

I added the ContextMenu to XAML and then set the Items like this (only one item here because it doesn'T matter):

<MenuItem Header="{x:Static Monitor:MonitorResources.R0206_SaveLatestValueToDatabase}"
                                      IsCheckable="true"
                                      IsChecked="{Binding ElementName=root, Path=Model.SaveToDbOneChecked}"
                                      IsEnabled="{Binding ElementName=root, Path=Model.SaveToDbOneEnabled}">

The SaveToDbOneChecked and SaveToDbOneEnabled are Properties in my Model which are implemented like this:

    private bool mSaveToDbOneEnabled;

    public bool SaveToDbOneChecked
    {
        get { return mSaveToDbOneChecked; }
        set { mSaveToDbOneChecked = value; OnPropertyChanged("SaveToDbOneChecked"); }
    }

I set these before the ContextMenu gets called on the SelectionChanged in the GridView the ContextMenu is in. But it won't show the Checked sign next to the text of the MenuItem although the SaveToDbOneChecked has been set to true! I don'T know where i do something wrong and hope that somebody can help me here.