Plaster mold not drying fully?

I've never really run into this problem but I'm making a two piece mold of a water based clay sculpture. The first half dried in a couple of hours, flipped it and did the second half and gave it about 5 hours, came back and the outside was still damp. Decided to let it sit overnight, came back to it and still the same feeling. It's now been 16 hours and its still damp and if you drag a fingernail across it the plaster comes off with no resistance.

There was no difference in method or material between the first and second half, I've got a heater on it but is there anything I can do other than that? Is this mold trash?

Plaster?
What kind of plaster? Plaster of Paris, Hydrostone? Hydro-Cal? reinforced with burlap? Cheesecloth? anything?

Gypsum (plaster) doesn't "Dry," it "Cures." So if it is soft to the touch, something went wrong with the curing process: mixing, water ratio, temperature.

Did you mix it really thick, so that you could trowel it onto the form?

How deep is the soft layer? (ie how deep can you scratch the surface? to the model?)

If it is still soft and 24 hrs, you may need to remove it and do over. the clay sculpture could be damaged in the process, and some resculpting could be necessary.

In my experience, stone molds always destroy a clay sculpt, so this may need to be resculpted if there is a full cure issue. Be sure to post pics when it comes appart.

Plaster of paris retains moisture for several days after it's cured. It will also leach moisture from the water based clay inside. If the the plaster is hard but feels damp, it could be fine. If it isn't hard, you are probably SOL.