Vel adjustment whell

Could be that the spring is preloaded too much and the force is causing “valve bounce” or a rebound effect as it bottoms out with too much force and bouncing closed faster, killing the amount of air by reducing the dwell time that it remains or can hang open. Especially as the pressure begins reducing and there is less force against the valve to overcome.
It commonly occurs often to the uninitiated, as many have a tendancy to try and increase the power wheel in an effort to compensate for the reduction in pressure as air is consumed. Seemingly logical to increase the powerwheel in an effort to maintain a high velocity as pressure is reduced. Eventually the balance of force needed to overcome tank pressure is exceeded to the point that the valve bottoms out violently enough to bounce shut quicker. With a stock valve and hammer at normal pressure , you likely havent too much to gain past halfway. Of course…. YMMVq Christmas tree

Try to give us a little more info to be able to help you out. What caliber, how many shots at each power wheel setting, fill pressure etc.
Air Force guns have a “sweet spot” which is (usually) around 2700-2800 psi. If you filled to 3000psi and got a lower number, the gun may have to “settle down” before it hits the pressure it likes.
Try filling to a lower initial pressure and shooting a complete shot string. Let us know what your results are.
Wildfire :5: