What may have happened is the clay has not cured all the way to its core; that is, the outside shell was hard baked but the inside clay was only partially baked. Over time partially cured clay will soften and 'eat away" at cured clay. To keep that from happening, the inner clay must be completely cured.
The solution to this problem is to use a technique called Series Baking (be sure to see our Index for earlier posts about the topic). Series baking simply means hard baking the inside clay before adding another layer (or multiple layers) of raw clay.
Many doll artists know baking a solid figure in one bake is risky. The way they achieve completely cured clay is by covering the doll's armature wire with clay and then hard bake that before adding a layer of raw clay.
TIP: Be sure to click on the Index topic 'Fixing Problems" for more information about common issues doll makers have.