Well, I don't know if this is what caused it or not, but I noticed a little icon by my clock that had been installed with the video card driver, called "ASUS GamerOSD". Clicking on this icon opened a list of two commands: Settings and Exit. Clicking Settings brought up a window displaying information on... making screenshots and video captures, complete with hotkeys. I tried this, and lo and behold, I had screenshots of my game.
Hopefully this will help anyone with the same problem in the future. Issue resolved!