To free up a stuck lock core spray the inside of the core with WD40 this should free them up. Once unlocked remove the cores clean them and allow them to dry. Then re lube the core with either a graphite or like lubricant. Do not use too much since a buildup of anything in a lock core can hinder the movement of the internal parts. Like the rest of your rack your cores will need to have some preventive maintenance done yearly to keep them turning freely.