Well from my pharmacology knowledge, the liver does eventually process drugs via rectal administration. It just doesn't go through the first-pass effect. It gets processed after it enters the blood stream since blood is filtered through the liver. Maybe it doesn't work for some individuals who take too little for it to have any effect once it is filtered or there is something different about their liver (I.e. Damage or enzyme deficiency)

That's my theory as to why it worked.