I have a compressor / 30 gallon receiver tank I have had for a long time. I haven't had success with an impact wrench for lug nuts.
I think it is because I have a pretty long hose on it and have too much pressure drop at the wrench, but I haven't tried to shorten it. Maybe I will as I get older and less fond of using the breaker bar.
Sounds like the numbers are a pressure adjustment, to allow you to dial down to less than full pressure.
It should work when applied normally to the nut, not necessary to press for any clutches or anything. Does it sound like it is impacting -- hammering -- when operated ? Or is it stalled, with just air exhausting ? If it's new, maybe it's broken ?
You should have a special socket for impact use -- thicker, malleable, less likely to fracture.
I may run the nuts on with the impact, but I always tighten to the final value with a torque wrench anyway.