no video but audio using wpf mediaelement

Question

I have a richtextbox in my wpf application and I am adding videos to it by using mediaelement. Then I convert flowdocument to xaml and store it to database. When I fetch xaml from database and convert it back to flowdocument and click play button, I can hear audio but cannot see video. Loadedbehavior is set to Manual.

Strange part is when I set loadedbehavior to Play, I can see video.

P.S. I am not good at english. so please forgive me for that.

Following is the XAML AND Source:

  <RichTextBox 
    Width="779"
    Height="200"
    IsDocumentEnabled="True">
    <FlowDocument c:FlowDocumentBehavior.DocumentResourceName="inlineTemplate" c:FlowDocumentBehavior.DocumentSource="{Binding VignetteTextXaml}"></FlowDocument>
    </RichTextBox>

Following is the code where I convert xaml string to flowdocument:

FlowDocument doc = d as FlowDocument;
        RichTextBox rtb = doc.Parent as RichTextBox;
        string xamlString = FlowDocumentBehavior.GetDocumentSource(doc);
        string templateName = FlowDocumentBehavior.GetDocumentResourceName(doc);
        if (xamlString != null && templateName != null)
        {
            StringReader stringReader = new StringReader(xamlString);
            XmlReader xmlReader = XmlReader.Create(stringReader);                
            if (!string.IsNullOrWhiteSpace(xamlString))
            {

                doc = (FlowDocument)XamlReader.Parse(xamlString);
                rtb.Document = doc;
                rtb.IsDocumentEnabled = true;
            }
        }

Only audio works but no video. :(

Thank you in advance.