In most places, fingerprinting is outsourced and done by a third party, who are not USCIS employees. Obviously, that also means that they are much nicer and a lot more friendly and helpful. I walked in for my fingerprinting a few days before the actual appointment in Dallas and it was fine. I think most of them are open Tuesday through Saturday instead of Monday through Friday. You should just walk in and see if they would do it even though you missed the appointment. Let us know what happens. Good luck!