Keystrokes are not visible when another app is in fullscreen mode
You are using another app in full-screen mode, but Keystroke Pro does not display keystrokes.
Go to the app settings, open the “General” tab, and check which option you have selected under “Show App in”. Displaying keystrokes over apps that are in full-screen mode only works if “StatusBar” (and not “Dock and StatusBar”) is selected there. Unfortunately, this is a technical limitation for which there is currently no other workaround.
Another approach is to simply scale the desired window to the full size of the screen. This also has the advantage of keeping the menu bar at the top of the screen permanently visible.