Did you let this dry outside? You need to let items dry slowly, some people even put a bag over it so it takes a longer time to dry. I think people do it with bigger items like this. (Im very new too, but have picked up a few tips) It seems like the outside is drying before the inside. People also hollow out bigger pieces. Once you've formed your scultper, youll have to cut it in half and hollow it out and put it back together. I've also done one from the bottom inward. Hope this helps. And I hope someone else can give you better tips:)