A few weeks ago, I had a lot of buildup in my hair. I used Herbal Essences Smooth Collection Shampoo (silicone free) and the buildup went away. Then I went back to my normal shampoo. Now I'm noticing it again, my scalp is very flat. I used the same shampoo again and my scalp is still flat. It's like it didn't get rid of the buildup this time. What's the problem?