Afro Busts

Someone pleeease tell me how you screw up Jello? Isn’t that a foolproof foodstuff? I set out to make these gorgeous afro couple heads, followed the instructions on the box and instead constructed a slime filled Nile River. The only thing I did that wasn’t on the box was to use a mold meant for plaster. But what’s the difference? It’s still meant to hold lquidy stuff into shape til it gels. Does the white plastic stuff the molds are made off prevent the normal chilling process from occurring? And what’s the deal with the raisins? They all floated to one level as opposed to spaced for lifelife hairs tufts. This was going to be my big pot luck dish this year so I am totally dismayed. Any illumination would be most appreciated.