The chocolate won’t stick to the toffee if the surface is too oily when the chocolate is added. To remedy this, be sure to wipe the surface with paper towels and sprinkle with cocoa powder before spreading with melted chocolate. Temperature can also cause the chocolate to come off. Be sure the toffee is at room temperature before adding the melted chocolate and before cutting or breaking into pieces. If refrigerating to set the chocolate, bring the chocolate to room temperature before placing in the fridge.