WPF popup is going behind when MainWindow Topmost is true

Hi All,

  My scenario is,
       Main window's Topmost is set to true
       WPF popup's IsOpen=true and StayOpen=true

   I want to display the window with popup ,but popup  is going back sometimes
  also  when user clicks on the window popup wont go back.

 I am calling the mainwindow.show() from another window
and i also used mainwindow.activate() also.

Is there any way to do this,Please suggest me,

Thanks,
Gowtham