I "think" the shrinkage is caused by it getting too hot when setting up so you might have added a bit much of the hardner...??? There are soooooo many varables when it comes to working with that stuff it can sometimes become a nightmare. I have changed over to Envirotex because even I can figure out a 50/50 mix
As far as fixing your issue I would either add more art water or mud, which ever would be easiest in your case. Did you make a deep pour ? I know that usually works against you when getting in a hurry and wanting to just do it all at one time. Only pouring about 1/4" max per time seems to be the best route.