Mold putty not curing!

Hello,
Has any one had issues with "Amazing mold putty" the two part yellow type not curing?
I have tried a few of times, mixed as per instructions with clean hands then pressed onto a cleaned hard styrene object.
Room temp in the mid 20s c, low humidity. Summer here so windows open etc though not direct sunlight each time I made a batch.
I have left it in the sun and under a warm desk lamp hoping it would aid curing but to no avail.
I also premixed all of each component incase it had "settled' in storage.
It seems readily pliable, slightly oily to the touch and combines well, it seems as I would expect from the videos I have seen of its use. 
Latest batch has been sitting for over a day still as soft as when it was mixed. 
All attempts have failed to cure.
I just purchased it from a local online retailer. I am not sure how old it is, no production date on packaging.
Does it 'go off' with age? The containers were sealed when I received it.
Is there an easy way to clean it of my model? A gentle solvent etc to help remove it from the detail it is now clogging?
Or is it just toothpicks and soft tissues again...


Any help is appreciated.

Thanks.