Problem with switching windows of same application

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http://askubuntu.com – I am new to Ubuntu and I just installed ubuntu12.04 for a week. Everything works fine until this afternoon. Normally, 2 windows of the same application are open, there are 2 grey arrows beside the icon on the panel. When click on the arrows, all the windows from the application line up on window for me to choose. But when I click on it now, nothing happened. The windows are not lining up for me anymore. I though that maybe I mess with unity too much and broke it, so I tried to reset Unity by unity --reset and then go to .config/dconf/ to delete the user file. I restart the PC after that. (HowTos)