I set the maximize keyboard shortcut to <super>up. after I set it it works fine. After I reboot the shortcut doesn't work. When I go to settings dialog the shortcut <super>up is still displayed. I have to enter the combination again for it to work.
So basically in order to have a functinnal maximize kb shortcut I have to go to the WM settings everytime I boot to set it again.
Other WM shortcuts seem to be remembered.