You might not be attaching to the correct process? Some games run with
multiple processes. Also, make sure you're in the game lobby before
attaching the process. In case you don't fully understand how it works,
I'll tell you. Check to see how much ammo you have in a clip. Put that
number into the section that says value. Press first scan and let it scan.
Back in game shoot another bullet. Go back into cheat engine and
enter the new number and press "next scan". Repeat this until you are
down to one address on the left. Double click that address and it will
go down to the box below. Now you should be able to change the value.
This thread also might be more useful if it was found in the "Hacking & Help"