Slitherine

Steam game version 2.2.5.

I was unable to take screenshot of the game if it is NOT in the Windowed mode.
If game is in Windowed mode I was unable to use F9 button listed under Controls tab under Options as the one for taking screenshots. Windows built print screen worked instead.

Did you remember to activate the Steam overlay by right-clicking on the game in your library and going into properties?

I just took a screenshot on Steam in fullscreen, so either it's your installation and/or system, or you forgot to do something.

Looking at it Steam overlay is enabled. I do remember I was able to take screenshots with F9 button in some of previous game versions.

I took the screenshot by hitting F12.

You are right, F12 works in full screen mode for me. It even gives notification that image was saved.

What folder do screenshots get saved in when they work?

Steam or non-Steam ?

Screenshots taken by pressing F9 should be stored in <my documents>/My Games/Order of Battle - Pacific/Screenshots