IF the vitamin C didn't work, then I am not sure what is causing them either. Have you tried to put a trichlor puck on them? If chlorine lightens them, then it would be organic in nature. The only other thing I can think of is if the plaster is mottleled. The usual way to tell if the stain is from metals is the vitamin C test, or you can try some ph down in a sock on the stain too. If both of these don't work, then my guess would be it is something with the plaster. Welcome to the forum, I wish I could help more. Maybe someone else will be able to give some more advice.