6 month old not rolling over yet,does not try to lift himself off the floor when on his stomach,does lift his head,also when someone picks him up ; he arches his back when holding him over your shoulder arches his back and holds arms stiff,other developmental milstones are normal. his muscles are not stiff when lying down, kicks his legs and reaches with his hands normally. why is he not rolling over and stiffens and arches his back frequently not just when eating?