WPF - ControlTemplate as Thumb control Template data binding - wpf

Question

I have a custom MyThumb user control which implements Thumb class. This user control is templated using ControlTemplate which ItemsSource is bound to a ObservableCollection of my custom class with three properties. ObservableCollection is in public property in code behind named ImportFileColumns and it's populated before InitializeComponent(). Problem is, that ObservableCollection is populated with correct data, but ControlTemplate never gets data bound (TextBlock control should be populated using values from ColumnName property), so my assumption is that I'm doing something wrong in the XAML.
Here's XAML:
<ControlTemplate x:Key="columnsThumbTemplate">
<Grid Width="175" >
<ItemsControl Name="isTest" ItemsSource="{Binding ImportFileColumns}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Orientation="Vertical" IsItemsHost="True" />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Grid Width="175">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="150" />
<ColumnDefinition Width="25" />
</Grid.ColumnDefinitions>
<Border BorderBrush="LightGray" BorderThickness="1,0,1,1" Background="White" Padding="10,5" Grid.Column="0">
<StackPanel Orientation="Horizontal" >
<TextBlock Text="{Binding Path=ColumnName}" Height="13" />
</StackPanel>
</Border>
<my:MyConnector x:Name="myRightConnector"
Template="{StaticResource tplRightConnector}"
Grid.Column="1"
IsConnectorClicked="False"
PreviewMouseLeftButtonDown="myConnector_PreviewMouseLeftButtonDown"
Canvas.Left="Auto"
Canvas.Right="Auto"
DragDelta="onDragDelta"/>
</Grid>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</Grid>
</ControlTemplate>
<my:MyThumb x:Name="myCustomColumnsThumb"
Canvas.Left="272"
Canvas.Top="400"
Template="{StaticResource columnsThumbTemplate}"
DragDelta="onDragDelta"
PreviewMouseLeftButtonDown="connector_PreviewMouseLeftButtonDown" />