Of course anything is possible and this is only 1 case I have found; I am a beginner of jujutsu currently, but 100% avidly dedicated. I have applied the ankle lock to 3 different people. One of which claimed don't even try, he's immune.... I was secured behind his achilles and mutilating his foot down, and had no result... I even moved into locking him down with my leg for extra pressure, and still no result... so I went on to another option. Does this happen often? Or am I possibly just executing it wrong?