ps -- A guy in the happy situation of being faced with a naked nipple is not going to give a hoot if it sticks out or not.
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.) It's only when you are older (or have breast-fed a baby) that the nips stick out all the time.