I overheard this the other day, while in a pet store, from a woman holding a prong collar in her hand: "It's not working. My dog is still pulling".
Most people who know me are well aware of my feelings about prong, choke and shock collars. I am not a fan of them. I do not use them or promote their use.
Punishment based training focuses on punishing the dog for unwanted behaviour when it occurs. The dog pulls – you apply the shock or jerk the leash to apply pressure so the prongs will dig into the neck. The dog wants to avoid the pain and will stop whatever behaviour caused it to occur.

Properly applied physical punishment works. I won’t say it doesn’t. The key words in that sentence are “properly applied punishment”. 
Pat Miller, CPDT-KA, CBCC-KA, owner of Peaceable Paws Dog and Puppy Training, lists 12 pitfalls of positive punishment (the dog’s behaviour causes a bad thing to happen).
The pitfall I want to mention is:  in order for the correction to be effective it must be strong enough to only be applied one or two times - and never has to be done again. It's not, one or two times every day.

In order for the punishment of a prong collar to be effective, the leash has to be yanked really hard. Many dogs develop a “punishment callous” because the person yanks the leash just a little bit each time (the dog gets used to it and it doesn’t bother them anymore). 
The punishment is not harsh enough to make a difference in the long run so the dog acclimates to the low level of pain. The next time the person has to yank the leash, he does it a bit harder because the dog stopped reacting to the first level of pain. And so on and so on. How are they supposed to know how hard to jerk the leash?
Too little and it’s ineffective. Too much and they damage the dog’s trachea.
Let me make something very clear. I am not proposing people use punishment to train their dogs. I have had great success with humane tools such as walking harnesses along with rewards based training, on breeds from Chihuahua’s to Great Danes to Leonbergers to Mastiff’s.
What I am saying is poorly applied punishment is what I see most often. It is ineffective, bordering on abuse in some cases.
I have seen strong dogs, on prong and choke collars, straining at the end of the leash because they have developed a punishment callous. 
So to answer the question: Why isn’t the prong collar working anymore? Because the average person is not skilled at applying positive punishment effectively. Frankly, I am glad for that in one sense. It tells me people don’t want to inflict pain on their dog in the name of training.  And yes, it is painful – or it wouldn’t work.