If the buttons don't work, you could always try using ADB from a computer. Root is not required but USB Debugging must be enabled. Then just run:
adb shell screencap -p /sdcard/screen.png
adb pull /sdcard/screen.png
adb shell rm /sdcard/screen.png
These commands will produce a screenshot and save it in the
sdcard directory (on some devices, including the Nexus line, have an emulated sdcard partition in the internal storage; so don't worry about this), pull to your computer, and then deletes it from your phone.
Unfortunately, one would think that this would be easier on stock Android. But that's why so many aftermarket versions or apps exist: to provide the missing functionality that people can cherrypick from and customize as they please. Below is an example of a simpler and nicer interface from which to take screenshots, which was added to stock 4.4.3 Android using GravityBox [KK]
(root and Xposed