Is the shampoo you use have some type of oil in the ingredients? When my hair gets oily/greasy (simply from my natural oils, they build up fast) I use a three step method. I use Tresemme healthy volume shampoo, then men's Axe deep clean hair scrub and then I deep condition. Always seems to work. Tlif it glfeels greasy several hours after washing its probably your shampoo and conditioner.
My husband noticed my hair is greasy too! Lol so he went to get his hair cut and asked a lady what he can do for my greasy pregnant lady hair!! Haha so she recommended tea tree shampoo and conditioner! Works miracles, my love!!