If they having up duck rather than wait for the force of rotation to drive them over, it sounds like they aren't driving from the heels enough. A drill I used (and what finally cracked this skill for me and many others....) uses two mats on the tramp, and is kinda hard to explain but here goes...
Put a big mat on the end deck, hanging down onto the back quarter of the bed. Put a small mat in front of it, in the middle of the bed, with a small gap in between the two. The gymnast stands in the gap, and from nothing (or from rocking) the jump up and push their heels back, kicking the big mat behind them hard. They can then return to feet (unlikely) or land on their front on the small mat in front of them. It forces them to keep the body straight because if they duck, they won't land comfortably, and they won't move the back mat, which is the aim. I did this with a group of kids and they all put markers of how much they moved the mat - made it more fun and effective as they all wanted to be the best!
Once they can do this confidently, they will understand how to drive the heels back more effectively (and hard). take the mats away and ask them to perform the same drive but harder, and they should be much straighter as they go round for the Crashdive.