Is this why my hair isn't curly anymore?

When I was younger, my hair used to be really curly. As I got older, my hair slowly got straighter, but the top was always straighter than the bottom. Flash forward, now i'm 16, and my hair is almost completely straight on top. However, the underside of my hair is still wavy (think loose waves/ringlets) and they start at around my ears. I've been trying to figure out why my hair is straight on top for years, and I realized something in the shower recently. When I shampoo my hair, I only shampoo my hair from around the ears up (where it has become much straighter), and rarely ever touch the bottom half of my head. Then, I put conditioner in the rest of my hair, which is basically all the parts that are still wavy. I have done this for years. Could this be the reason why my hair is so inconsistent? Would using a no-poo shampoo, and conditioning all of my hair help bring my curl back?

Thanks