probably your press is too hot, try to work wit 10 centigrades cooler and compensate with larger time,
if you have your press too hot, the rubber starts vulcanizing on the surface, that vulcanized rubber acts as a thermal insulator not allowing the heat to travel to the core.
in the R.T.Vanderbilt rubber handbook you can see what to check on each raw material, in Rubber plants you check the Mooney Viscocity of the raw rubber and some more like the moisture of every powder, etc