It's hard to go into specifics without seeing your pass but here are a few common issues:
1. Don't try and go fast until you've done a few thousand reps of your pass and it feels like a single smooth and natural movement, not two jerky movements.
2. Remember you're not trying to fool yourself. In other words, if you look down at your own hands and the pass looks invisible, it's probably flashing to anyone in front of you. So, you'll probably need to tilt your hands forward a bit more than your think so your audience can't see the work happening under the deck. Mirror practice helps to get this right.
3. Hold the deck slanting at about 45 degrees. You shouldn't be holding the deck perfectly horizontal or perfectly vertical.
4. The close is part of the motion of the pass. Squaring the packets together is as important as making them transpose. So after you've got the top packet to the bottom, the movement doesn't finish until the packets are completely coalesced. Don't pause with the deck unsquared.
5. Make sure you practise doing the pass in context. Hold a break in one hand, bring the hands together, execute the pass and then separate your hands. Make that whole process smooth with no fumbling as the hands come together or separate.