Pode ser um problema com o compiz, que você pode resolver ao reinstalar o compiz.
Remover o compiz:
sudo apt-get remove compizconfig-settings-manager sudo apt-get remove compiz-fusion-plugins-extra sudo apt-get remove compiz-plugins-extra sudo apt-get purge compiz*
Reinstale o compiz:
sudo apt-get install unity-2d sudo apt-get install ubuntu-desktop sudo apt-get install ubuntu-desktop-2d sudo apt-get install compizconfig-settings-manager sudo apt-get install xserver-xgl sudo apt-get install emerald sudo apt-get install compiz-fusion-plugins-extra sudo apt-get install compiz-plugins-extra sudo apt-get install unity