Suppressors work by trapping gas pressure and slowing its release into the surrounding atmosphere. In the case of the AR-15, using the standard direct impingement gas system, this causes increased pressure within the gas system of the rifle and blow back for the shooter.
Things that might help.
Use less lubricating oil on your bolt/upper. This will keep the gun cleaner to begin with and produce less smoke and/or oil to blow back in the shooters face.
Switch to a gas piston operating system