(I'm assuming you're talking about piking in the preflight)
Piking is often (though not always) caused by leaning to far forward on the board. Which is often caused by hurdling from too close. Which is often caused by problems in the run.
I'd start by looking at the run, and specifically by looking at where you start your run. Try starting a little closer, or a little further, or starting with the other foot, or some combination of these. The goal is to, without throwing off your stride, be able to hurdle from a bit further from the board.
If this isn't the problem (and it may well not be; I'd have to see you vault to say for sure), then focus on your hurdle. Try picking your chest up and pulling your chin back a bit on the last couple steps of your run, and keep it that way through the hurdle. If you are leaning too far forward (and again, I can't say for sure that this is your problem without seeing it), it is almost impossible to vault without piking (unless the board is set very far from the mat, something which I would avoid since it leads to a weaker vault over the table).
It is possible that you are vaulting with the board too close or the mat too high. I would hesitate to diagnose this problem over the internet, though; generally, you WANT to get used to vaulting with the board as close as you can reasonably get it.
You may be ducking your head under too early; coaches differ on whether your head should be neutral or out, but I think most will agree that you don't want your chin on your chest for this skill.
Or it could be none of the above; perhaps you just need to squeeze your butt and focus more on getting your heels up as you leave the board.