cannot make a main window modal

[Dark_Tower]

Hi, I have a main window that I initialize it at full screen. This window is child of another window. I want that this main window acts like a modal dialog window respect its parent. I've tried it setting its window modality to Qt::WindowModal or Qt::ApplicationModal but it doesn't work because when I minimize the main window, for example, I am still able to access to its parent window. Anybody knows how to make it?

Thanks.