I've noticed screenshotting on the 6 and 6 Plus are a bit trickier than before. You have to hit the power button first, then the home button. It's gotten me many times where it locked my phone without getting the screenshot.
I've noticed the same thing, it's much more difficult to take a screenshot with 8.0.2. I think stephen1108 summed it up well, if you hit the power button just a split second earlier than the home button, you can technically get it to work....but it's a B* trying to time it correctly.