Can't take screenshot on Pixel C

Just got a new Pixel C tablet. It's great. Very responsive and functional. However, screenshot does not work. Instructions say hold volume and power buttons. And that worked once, but I could not repeat it. Whatever I do it either pops up the power menu or changes the volume. I think I managed to get two screenshots out of 100 tries.

Did I forget to say "expecto patronum" or something? What's the trick? Sure do wish they would just make it work by holding down the switch apps button like previous things. That always worked on my previous tablet.