So, I can't really take any screenshots. At first I thought the problem was only happening with 2D games, because I couldn't take screenshots on Terraria, Limbo, or Braid, and I could take screenshots on Portal 2. But today I tried taking screenshots on Borderlands, but that doesn't work either.
The screenshot function works for sure because it takes a screenshot on Portal 2 when I press F12. But it doesn't seem to be taking screenshots when I'm playing any other game? What's the problem?
You can try using the free version of Fraps or Game Booster 3 for screenshots if the Steam solution isn't working for you. I use Game Booster because it also reduces the number of Windows services running while I play games so it saves memory despite having another application running in the background. Best of all, it's totally free!