QDialogs (sub) do not open sometimes

Mainwindow is 800x1280, subdialogs are 800x1100, all notmodal. When clicking button for subwindow, it opens and i can see the last 180 pixel of mainwindow, and use existing buttons in this area from mainwindow also. Sometimes it happens that subwindow is not shown, it seems as if nothing had happened after buttonclick. But if i click Menubutton in such a case and appIcon again (or reopen via taskmanager), the subdialog is on top directly, but now in size 800x1280, the buttons from mainwindow are no longer shown. The subdialog is defind in creator as 800x1100 and also maxsize is set to this value. It's opened with directive show.maximized().