Today, I had an interesting case where I couldn't take a screenshot of the Playground Buddy android app. I hunted the usual suspects and all confirmed that you had to press the volume down and the sleep buttons at the same time. I tried. over and over and over again. No luck. Eventually, I remembered that I had turned off the animations on the phone (this option is available under the Developer Options). So I turned them back on.

..and presto. I can take screenshots again!