PrintScreen key not taking a screengrab? Alt+PrintScreen not capturing the window? Using Gnome? And Compiz? – Fix it thus:
Right click on the Compiz-Fusion Icon (Don’t have? Install directions here), then right-click on the Compiz Fusion Icon and select Settings Manager, then from the settings window that opens find Gnome Compatibility under the General section and enable it by clicking the tick-box.