This was going to be my answer as well.
To everyone that told him to "practice more", practice is not always the answer. If he is performing it wrong then what good will practice do? When it comes to the clipshift there are 2 places where you can flash big, between the fingers and under your hand pinky side.
So what should have been asked was, Where are you flashing? Is it under your hand or between your fingers?
If you are flashing between your fingers make sure you are clipping the card the way taught on Surfaced. Also, make sure you are keeping your fingers closed. That is where the mirror will come in to play. If you are flashing under your hand (which is much more likely than flashing between your fingers) then the problem is more than likely coming from 2 things being done incorrectly, but can be fixed very easily. The link above from Jenai for The Magic Session is for an On Demand lecture with Chad Nelson. In this lecture he talks about the actual clip in the clipshift and bringing the shifted card closer to your palm.
I believe those are your problems. Watch the lecture and make the appropriate changes to make your clipshift invisible.