Hi.
My system:
Asus G75 with nvidia GTX-660M
Linux BM 3.11.2-gentoo
x11-drivers/nvidia-drivers-325.15
kde-4.11.1
qt-4.8.5
I have trouble
Desktop compositing randomly turning off:
after kde start all fine,
but, when i open, for example, konsole, all fine,
then i open another konsole (or type in first konsole),
and compositing is off!
To fix this i found change some settings in "Desktop compositing" (for ex: opengl-2.0 to opengl-3.1, or backward) and push apply in popup message window.
Please, help me fix problem.

pressing "shift+alt+f12" toggles the compositor
if you can rule out you did that, scripts and window rules can block compositing (for certain windows) but it should return when the blocking window goes.