All brands of polymer clay hardens upon storage. Manufacturer recommend kneading to reconstitute the plasticiser to soften the clay. If necessary addition clay softener or diluent for the needed project softness.
To soften, break hard clay into clumps and add by drops the softener and shake to disperse within a glass bottle. With a clay roller or pasta roller, roll in several passes, folding and rolling progressively narrowing the rollers gap width until a smooth soft sheet is obtained.