I think I got your problem.
Your index finger is very on top of the deck. In oder words, you want your index to hold the deck on the right side of the deck, not on the very top; you also want your finger to not do a lot of pressure to hold the cards; make sure you relax all your fingers (exept your pinkie) when do the move.
Hope not expose a lot on this post ...
Hope that helps.
J.