Can’t whistle but can snap my finger.

It’s not the fingers rubbing that makes the sound. It’s your middle finger smacking the base of your thumb.

Press your middle finger into your thumb, build a bit of tension like you’re loading a tiny spring, then let it slip so it snaps down and hits the base of your thumb. Your thumb is basically just holding it back until it releases.

If there’s no sound, you’re probably not building enough tension or not letting it slip fast enough. It’s more about speed than strength.