This happened after upgrade to 4.9. After KDE startup there's no slide effect when clicking on plasmoid. In some themes also blur doesn't work. Workaround is to kill plasma-dekstop, then turn desktop effect on and off.
I forgot to write that my graphics card is Nvidia GeForceGTX 550 Ti and I'm using lastest Nvidia proprietary driver (304.51).
smells like bug #179042 - at least plasma misdetects the compositing state. do you get some initial kwin crash or so?
No, there's no any "kwin was crashed" window after system start.
What happens if you suspend and then resume compositing (shift+alt+f12, wait a second, shift+alt+f12)? Pre-emptive marking as dupe. *** This bug has been marked as a duplicate of bug 179042 ***