What Causes Brown Spots On Neck, Underarm And Chest?

Hey doctor, im 16, and since I could remember, I've had brown spots or patches, in my neck, underarms, chest, and lower back, they feel like dry skin but I've been appling lotion on, so far it doesn't seem to be having an effect! It looks gross and Ive been to a couple of doctors and they seem to not have a clue to whats the problem, please help!!