Ok let's see if we can get things to work for you and figure out what's going wrong.
First, can you make sure you are using the latest version so we know we are talking about the same thing.
Second, and most import, is to make sure AS *can* capture the window you care about, and that the saved screenshot is correct -- that is, it shows the contents of the captured windows. Leaving aside the issue of whether it detects all changes or not.
If it does that, then there is no reason it shouldn't be able to do what you want.
Assuming that the above works, it may simply be a matter of AS not capturing/monitor what you think it is.
It sounds like the "Active window or desktop" is the setting you want. Note that this captures the currently active window on the desktop (the one receiving focus of the mouse/keyboard).
Set capture rate to 1 second for maximum capture speed.
Now, the option "Also capture whenever the active window changes" may be slightly misleading.. It has nothing to do with checking the content of the active window -- that should already happen automatically. It's simply saying you want AS to capture IMMEDIATELY any time the user selects a new active window.
Let me know the answer to those questions.
It may be that what you would benefit from more would be an option to tell AS to watch and capture only the window of a specific executable...
But let's first focus on getting AS to perform properly for you given the current options. The main reason I would think it might be failing is if you are switching away from the window you are about (making another window active), OR more likely, if you have the capture rate too high. Set that to 1 and it should be capturing once every second and evaluating the window for changes.
Try keeping the main AS window open and then select your active window and look in the AS window to see what it tells you it is doing.