Aww, I was gonna say torch it. Just kidding.
I have the same thing happening to me with my oil-based clays (non-hardening). The longer you leave the clay in the box without touching it, the generally harder it will get when you first start molding it. I would guess maybe microwaving it (I take no responsibilities for any damages) or keeping them in air-tight bags. But honestly, I've just accepted the fact that at first they are difficult to mold - just pound it a few times with a bat and it should be fine.

Hope this helps.

Don't put it in the microwave, it goes weird and smells awful. Newplast dries out a lot, I don't know any way around it. Also in the cold weather it seems to go hard and crack when you're out and the heating isn't on.
And yes, it does burn rather than melt 

Perhaps leave it in a somewhat warm room for a while? I guess you can't microwave so, that's really all I got.