i have 2 outlets in parallel that show a weak ground with a 3 pronged tester but they work fine for lamps and other electrical appliances. they are downstream from other outlets that test fine and i do not have any outlets that are dead. When I test voltage from hot to the box on these 2 outlets I can't get a reading but on the rest of the outlets i get a 120 volt reading. What do I need to do to correct?