You might not be putting enough pressure when you do it, and once you do have enough pressure, you have to make a quick motion with your thumb that flings it from the middle finger to the index finger
The wooshing noise is what becomes a snapping noise if enough pressure is applied
The sound is made by hitting the middle finger against the pad at the base of your thumb. Make sure that you are using as much force as possible and sliding your finger directly down the side of your thumb.
Oh patty-cakes, foiled again!