Isn't the very existence of positive law and its enforcement by the state an admission that natural law is either not universal or not efficacious?
A law is a rule and measure for human acts, not a force that compels them to act that way. Human beings cannot blot out the knowledge of the universal precepts of natural law, but they can certainly disobey them, just as they can disobey the laws of the state itself.
I might add two points. First, though human beings can disobey the natural law, they cannot disobey it with impunity. There are always natural consequences. There are even natural consequences for trying to evade the natural consequences. The birth control pill was supposed to make it possible to prevent pregnancies out of wedlock without practicing chastity, but its consequence has been a change in behavior and attitudes so radical that the rate of out-of-wedlock pregnancies has soared.
Second, in the fallen condition of human nature, we are confronted with not one but two universals. Not only is moral knowledge universal, but the determination to play tricks on moral knowledge is universal, too. A law is written on the heart of man, but it is everywhere entangled with the evasions and subterfuges of men. Even so that law endures; and even so it is seen to endure.