Window erasing background screen issue with service program

I have a window display program that, when called directly, the window
pops and the screen behind remains on the display. - All good.

When I call this same program from within a service program the window
pops, but the screen behind is erased; so all i see is the window, with
a blank screen behind.

The window operates fine in both circumstances, it is the erasing of the
screen behind the window that is an issue.

Is there a way to retain the screen data behind from within a service
program?

Am I, again, misunderstanding something?

Thanks for any help.

Paul