You've probably set the over travel screw in the trigger face too tight. The trigger will then go back just far enough to trip the sear, but not enough to lift the firing pin safety plunger out of the way. The firing pin is probably forcing it's way past the plunger, losing energy in the process. Back off the screw to check this. On guns with Series 80 style safety parts it takes a lot more work to set the over travel "short."