C# TextBlock drag and drop operation in WPF RichTextBox

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C#3.5

Please help with the following: In my WinForm, I have a WPF RichTextBox in which Paragraph (only one Paragraph block is used) may contain one or several TextBlock's. When I try to drag-n-drop any TextBlock to a different location within the same RichTextBox, it disappears at the source location but is not inserted at the target location. Instead just a space is inserted. I have no idea where to search. Most topics are dedicated to drap-n-drop operations between different controls, not within the same one.

<UserControl x:Class="TetranTranslationStudio.MyRichTextBox"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Height="162.711" Width="559.944" AllowDrop="False">
    <RichTextBox x:Name ="TargetBox" AcceptsReturn="False" SpellCheck.IsEnabled="True" Height="Auto" Width="Auto" VerticalScrollBarVisibility="Auto" IsDocumentEnabled="True" FontSize="14" FontFamily="Arial" AllowDrop="True">
        <FlowDocument xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" ColumnWidth="400" FontSize="14" FontFamily="Arial" AllowDrop="True">
                        <Paragraph>This is a sample string which contains
                    <TextBlock>TextBlocks</TextBlock> that disappear when drags to a different location within the control
            </Paragraph>
        </FlowDocument>
    </RichTextBox>
</UserControl>

I will very much appreciate any help.

Vitaly