Child window opens behind main window

Since a couple of months quite often (but not always!) when I click on a button which creates a child window (like selection of a file in a directory, or an alert etc) nothing apparently happens except that I cannot do anything on the main window.
Actually the child window has opened, but stays behind the main window!
So I have 3 choices: press Alt-F4 (which does'nt always work) or minimize all windows and then reopen them again (which does'nt always work either). The 3rd choice is to kill the program!
This may happen in nearly all programs.
I tried to solve this problem with Tweak UI (Prevent application from stealing focus) but it didn't work.
Any suggestion?
Thanks
Hors