I would also recommend a variation of technique. A big part of tickling is sensation and different speeds, pressure, tools, etc add different sensations. If I were to guess, I would say that you were losing sensitivity to that particular sensation. My current girlfriend is very ticklish on her feet, but not to typical nail sputtering or soft caressing. The most effective thing we found was our Violet Wand, because it's such a unique sensation. So, it's just a matter of finding the different sensations that work and switching it up to keep you from anticipating and losing sensitivity. Hope it helps! ^_^