Visible cracking and signs of shrinkage with a newly made latex mold is usually associated with the mold being exposed to excessive heat for a long period of time (>150°F for 6 hours or more).  Excessive heat also tends to prematurely seal the surface of the latex film, thus trapping moisture inside, which in turn affects the cure and strength of the finished mold.  It can also result in delamination between layers.

If you are heat curing your layers of latex to accelerate dry time, you want to keep the heat setting between 120°F-140°F being sure to remove the mold from the heat source when the latest layer shows signs of being cured.