screenshot issues

I know the two methods of taking a screenshot -- karate swipe and power/home key -- but I have some apps in which neither will work. Is there a reason this could be the case? I sometimes play games on my phone and would like to take a screenshot of my score or whatever. But when I try eithr method, nothing happens and nothing is saved. Could it be a licensing issue, in which the developers won't let you take images of their games? Seems silly, as there are so many other ways to get an image of it (take a pic with another phone, for example). Just wondering whether there's some kind of trick to taking the shot within the apps.