Call modal screen in modal screen with jscript

[newuser]

HI,
I have a window with several screens.
One of the button calls a modal screen. (callbackresponse.addjavascript(…..))
When I call from this window a new modalscreen (same way) this screen sometimes appears in front but with other buttons behind the first modal.
It is alway the same button..
changing the button does not help
z-index does not help