Tab Index bothering between wpf Textbox and User control

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C#

WPF

C#4.0

Hi i have a wpf user control and wpf textbox control in a page, when i pressed on Tab in keyboard it's not going to next control which is a normal textbox. i had set the property tabstop to false and even true but nothing worked...

i have searched all the way but i can't find the solution, kindly give me a better solution to do this.

<StackPanel Orientation="Vertical">
      (User Control)
      <controls:UserControl x:Name="txt_Name" Header="Name"/>

       (Normal TextBox Control)
      <StackPanel Orientation="Vertical" Margin="10">
          <Label Content="Size" VerticalAlignment="Top" />
          <TextBox Name="txt_size"/>
      </StackPanel>

       (Normal TextBox Control)
      <StackPanel Orientation="Vertical" Margin="10">
         <Label Content="Age" VerticalAlignment="Top" />
         <TextBox Name="txt_age"/>
      </StackPanel>

        (User Control)
      <controls:UserControl x:Name="txt_primaryActivity"  Header="Primary Activity"/>
  </StackPanel>