focus() is not switching

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C#

On selecting the Enter key the system should do the following and switch focus to

txt_Interest.Text;

However it doesn't. How do I correct this?

Thanks

protected void txt_Principal_TextChanged ( object    sender, 
                                           EventArgs e)
     {
     decimal Kprincipal = Convert.ToDecimal ( txt_Principal.Text );

     txt_Principal.Text = Kprincipal.ToString( "N" );
     txt_Interest.Focus ( );
     }