Where are you writing your messages?
You should write your message in the PBO of screen 9000 if you want it to appear there. Hope its done that way.
Also just comment the set screen statement.
call screen 9000.
in the PBO of 9000, write the code to display your message
In the PAI.
when 'BACK'.
leave to screen 0.
will take you to the previous screen.