I'm 19 and I have an undercut (half my head is shaved from my neck to right behind my ears) I shaved it VERY short and its been a few weeks and a part on the hair wont grow in (fine hairs but its not growing in thick and dark like the rest of my hair.) I have thick Spanish hair so I don't have light or thin hair, but it wont grow in and its from the neck up about an inch. in more of a triangle shape.