Sounds like my hair. I've always struggled with trying to get my back/underneath to match the top/sides. For me, the key is HOW I apply the product more than the product. I rake my product in all over, using my fingers like a wide-tooth comb, and squeezing my fingers together with the hair in them to pull my hair down. Then, when I'm done with that, I scrunch the top/sides to encourage curl more there.
I'm a 3b, with 3c underneath.
Used to be CG, now I'm not. Using Suave Aloe & Waterlily shampoo & condish, JoiWhip, BRHG, Suave Aloe gel, Aussie Tizz No Frizz, and anything else that looks good!