I have the exact same problem occasionally. Sometimes F12 works to take a screenshot and sometimes it doesn't. What I have found is that after my game has launched, while I am on the START game screen, when I press Shift+Tab, to open up the Steam overlay nothing happens. On those occasions I will not be able to use F12 to take a screenshot. If I quit the game & restart, 100% of the time I will be able to access the Steam overlay by pressing Shift+Tab, and F12 will also work to take a screenshot while playing. This may be unique to my setup, but I thought I would share this information with you hoping it may help.