I have a harbor freight earthquake impact wrench that I used commercially in a small engine shop for 5 years straight. Its old, one of the first ones available, probably has 700 or 800 ft lbs of torque. And it will absolutely, positively bust off every nut I have ever tried it on in seconds. If the nut doesn't come off, then the bolt is getting shattered. But that bolt is coming off, guaranteed.
I dont know what impact you are running, but might be time to upgrade. Even the cheap earthquakes are incredible, for the money.
Hopefully some idiot didn't put red loctite on it.