If the doctor said you are fine, it might be that your nipples function perfectly normally. When I was a teenager especially, and into my 20s, my nipples would be "not erect" most of the time. Does the tissue around your nipples contract [so the nipples themselves stick out] when you get cold? (Back in the day, that used to be how they would get the Playboy models' nipples to stick out for photo shoots -- they would touch them with ice cubes.