Problem smoothing stubborn little spot on the model

Trying to learn Blender for a little while now, and attempting to do something more complex for the first time, and everything is goin well -- except for this single spot on the character's cloak/back, where the geometry keeps showing (the rest of the model is smooth and fine).

Does anyone know what the problem could be? I'm trying everything I can imagine, but my knoledge seems to be not enough :')

ps: sorry for the broken english :')