YOGO Milk Jam 3D staying soft after curing

I never used any clay/3d gel but really wanted to try. I bought this YOGO Milk Jam 3D but I noticed that it remains soft/rubber like after curing. I don't know if it's normal or it should be rock hard ... Any tips?

I never used any clay/3d gel but really wanted to try. I bought this YOGO Milk Jam 3D but I noticed that it remains soft/rubber like after curing. I don't know if it's normal or it should be rock hard ... Any tips?