Do you change pressure between holding the cards and smoothly transitioning packets around or do you apply constant strength between all your actions? Sometimes I find myself not gripping packets hard enough between my fingers, and one or two cards slip out even though I have a skin-cutting grip on the rest of the cards of the packet. It's probably positioning when I go to grip the cards before the move, like when you play the acoustic guitar and you don't position your fingers correctly over the strings and it makes a 'pluck' noise instead of a resonating note because you're not pressing on the string hard enough or your fingers are in the wrong position.