My hair is just so strange sometimes. I can thoroughly wash out my hair and not put in a single product, but there's this pie piece section at the back of my crown that always feels "gummy". I remember telling Tammy of a Not A Rich Girl this, and she suggested leaving your shampoo in your hair for a moment.
I always leave my conditioner in my hair for a minute or two, but I never thought to do so with my shampoo. I leave my shampoo in for about a minute before washing it out, and it works beautifully! So if you have super oily hair or get greasy patches like I do, try leaving your shampoo in while washing your face and see if it works!