Text property does not update

[Quin]

I have a multiselect control that I created using the RadComboBox.  I want the Text property to be updated to display the CheckedItems.  The ToolTip works fine.  It is updated whenever CheckedItems source changes.  However, I cannot get the Text property to update. The dependency property, "PreviewText", is a comma seperated string of the selected items.  The binding to the text property works the first time that the source is set, but does not update when the source changes.

<Style TargetType="ccb:CheckableComboBox">
       <Setter Property="Template">
           <Setter.Value>
               <ControlTemplate TargetType="ccb:CheckableComboBox">
                   <Grid x:Name="LayoutRoot">
                       <telerik:RadComboBox x:Name="ItemsDisplay"
                                            VerticalAlignment="Top"
                                            Text="{TemplateBinding ShortPreviewText}"
                                            ToolTipService.ToolTip="{TemplateBinding PreviewText}"
                                            ToolTipService.Placement="Top"
                                            CanAutocompleteSelectItems="True"
                                            IsTextSearchEnabled="True"
                                            IsEditable="False">


                       </telerik:RadComboBox>
                   </Grid>
               </ControlTemplate>
           </Setter.Value>
       </Setter>
   </Style>

Also, I tried this:

<telerik:RadComboBox x:Name="ItemsDisplay"
   VerticalAlignment="Top"
   Text="{Binding RelativeSource={RelativeSource Self}, Mode=OneWay, Path=(ToolTipService.ToolTip)}"
  ToolTipService.ToolTip="{TemplateBinding PreviewText}"
   ToolTipService.Placement="Top"
   CanAutocompleteSelectItems="True"
  IsTextSearchEnabled="True"
   IsEditable="False">
                       </telerik:RadComboBox>