You may need to scroll it more than once or twice for full throttle. If you can activate WEP, war emergency power, p key, and it boosts the manifold pressure some you are at full throttle. Just make sure you are in a plane that has WEP available.
You could also check in the 'map controllers' area on clipboard. Look at the numbers next to scroll wheel, throttle, and see if they max out and can go no farther.