Although this may sounds silly to point out and perhaps unneedy to say, but please make sure your TG Murphy Deck Flip is correct. I understand it is seemingly pointless to make a statement about, but really do make sure -- there are a lot of people out there who have a wrong technique for this move; such as the deck flipping the wrong way (it should flip from the thumb) and even the wrong hand flipping the packet.
Once you have this, ensure you have a consistant Deck Flip before attempting the production. Also make sure that the left hand packet is pretty well squared before performing the Flip -- any unsquare cards will cause the air to go elswhere (other than beneath the top card). I should also mention, do please experiment with different crimps - perhaps even with a pinky break if necessary. Not everyone handles a deck of cards in the same way, and different crimps work better/worse for different people.
Once you have these small factors, continue practicing the production until you can do it consistantly; even when blindfolded.
Warm regards,
Jordan