Off the top of my head I'd say it's a mag issue or maybe you don't have the grip safety depressed when racking the gun. Could also be that the safely is malfunctioning in some way. Perhaps the fully loaded mag is putting too much pressure on the slide to rack properly. Try it with different mags after you clean and lube the gun.
If you are still having the issue then call SA and return it for maintenance.
As for a temporary workaround, when loading the gun lock the slide to the rear and load via power-stroke.