I'd have to see to be sure, but my guess is that you're leaning too far forward on the board. This tends to slow your rotation (ie kill your heel drive), and cause you to come in too low (forcing you to bend your arms).
Try to pop your chest up and slightly back as you take off on the hurdle, and get your feet in front of you. Sometimes it helps to think about pushing the board forward with your feet as you hit it. You may need to adjust your run so that you can hurdle frim slightly further away from the board.
hey have you tried handstand pops on the floor?
if not its a good way of getting your hands off the vault quick enough so as ther is no time for your arms to bend, also general strength exercises should help, for example, push ups and handstands.