UpdateSourceTrigger=PropertyChanged does not fire within user control.

I have the following usercontrol. 

<UserControl x:Class="CommonControl.ctlTextbox"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:system="clr-namespace:System;assembly=mscorlib"
             mc:Ignorable="d" 
             d:DesignHeight="23" d:DesignWidth="300"
             Margin="0,1"
             x:Name="userControl"             ><DockPanel LastChildFill="True"><TextBlock DockPanel.Dock="Left" x:Name="label" TextAlignment="Right" Padding="3,0" VerticalAlignment="Center" Text="Title:"/><TextBox x:Name="textbox"                     
                    Text="{Binding ElementName=userControl, Path=Text}"
                    MaxLength="{Binding ElementName=userControl,Path=MaxLength}"
                    IsReadOnly="{Binding ElementName=userControl,Path=ReadOnly}"
                    IsEnabled="{Binding ElementName=userControl,Path=IsEnabled}"                
                    Validation.ValidationAdornerSite="{Binding ElementName=userControl}" 
                    ToolTip="{Binding ElementName=userControl,Path=ToolTip}" ></TextBox></DockPanel></UserControl>

public partial class ctlTextbox : UserControl
    {
		#region Fields (6) 

        public static readonly DependencyProperty EnabledProperty = DependencyProperty.Register("Enabled", typeof(bool), typeof(ctlTextbox), new FrameworkPropertyMetadata() { BindsTwoWayByDefault = true, DefaultValue = true });
        public static readonly DependencyProperty LabelTextProperty = DependencyProperty.Register("LabelText", typeof(string), typeof(ctlTextbox), new FrameworkPropertyMetadata() { BindsTwoWayByDefault = true });
        public static readonly DependencyProperty MaxLengthProperty = DependencyProperty.Register("MaxLength", typeof(int), typeof(ctlTextbox), new FrameworkPropertyMetadata() { BindsTwoWayByDefault = false });
        public static readonly DependencyProperty ReadOnlyProperty = DependencyProperty.Register("ReadOnly", typeof(bool), typeof(ctlTextbox), new FrameworkPropertyMetadata() { BindsTwoWayByDefault = true, DefaultValue = false });
        public static readonly DependencyProperty TextProperty = DependencyProperty.Register("Text", typeof(string), typeof(ctlTextbox), new FrameworkPropertyMetadata() { BindsTwoWayByDefault = true});
        public new static readonly DependencyProperty ToolTipProperty = DependencyProperty.Register("ToolTip", typeof(object), typeof(ctlTextbox), new FrameworkPropertyMetadata() { BindsTwoWayByDefault = true, DefaultValue="" });

		#endregion Fields 

		#region Constructors (1) 

        public ctlTextbox()
        {
            InitializeComponent();
            Binding labelTextBinding = new Binding("LabelText") { Source = this, Mode = BindingMode.TwoWay };

            label.SetBinding(TextBlock.TextProperty, labelTextBinding);
        }

		#endregion Constructors 

		#region Properties (8) 

        [Browsable(true)]
        public new bool IsEnabled
        {
            get { return (bool)GetValue(EnabledProperty); }
            set { SetValue(EnabledProperty, value); }
        }

        [Browsable(true)]
        public string LabelText
        {
            get { return (string)GetValue(LabelTextProperty); }
            set { SetValue(LabelTextProperty, value); }
        }

        [Browsable(true)]
        public double LabelWidth
        {
            get { return label.Width; }
            set { label.Width = value; }
        }

        [Browsable(true)]
        public int MaxLength
        {
            get { return (int)GetValue(MaxLengthProperty); }
            set { SetValue(MaxLengthProperty, value); }
        }

        [Browsable(true)]
        public bool ReadOnly
        {
            get { return (bool)GetValue(ReadOnlyProperty); }
            set { SetValue(ReadOnlyProperty, value); }
        }

        [Browsable(true)]
        public string Text
        {
            get { return (string)GetValue(TextProperty); }
            set { SetValue(TextProperty, value); }
        }

        [Browsable(true)]
        public TextBox TextBox
        {
            get { return textbox; }
        }

        [Browsable(true)]
        public new object ToolTip
        {
            get { return (object)GetValue(ToolTipProperty); }
            set { SetValue(ToolTipProperty, value); }
        }

		#endregion Properties 
    }
}

This user control is then used in a window as follows:

<Window x:Class="Server.frmLogin"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions"
        xmlns:cc="clr-namespace:Server.CommonControl;assembly=Server.CommonControl"
        Title="Login" Height="200" Width="320" 
        DataContext="{Binding Source={StaticResource Locator}, Path=EmployeeLoginViewModel}" WindowStartupLocation="CenterScreen" WindowStyle="None" Topmost="True" ResizeMode="NoResize"><DockPanel><cc:ctlTextbox Name="txtUsername" DockPanel.Dock="Top"  LabelText="Username:" LabelWidth="80" Text="{Binding Username, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True, NotifyOnValidationError=True}" 
    MaxLength="25"  /></DockPanel></Windows>