Having charging issues

Well my 68 Darts battery keeps dying. I know it's because it isn't getting charged, when I take it to oreillys there tester gives my battery alternator and starter a pass, but it always says that the voltage regulator has failed. Everytime. I changed it out right there in front of them and it still says it failed. Any ideas?

68 dodge dart 318 4 speed new battery alternator starter and pretty much everything electrical is new.