Because the syrup is undercooked. Amber walnut kernels, that is, fossils formed by imitating insects wrapped in resin, are boiled with sugar to imitate resin and wrapped in crispy walnut kernels to form transparent "walnut amber fossils". However, if the amber walnut kernel is not sticky to the touch and sticky, crisp and sweet to the mouth, it is necessary to cook the syrup correctly. The syrup is best boiled with rock sugar, not with sugar or honey. Remember not to use tools during cooking.