IF her step is on her takeoff mark each time, lower her grip 3". Too high of a grip can be a signifier of stalling out, as are many other faults in a vault.
How are her arms positioned at takeoff? If her arms aren't fully extended giving her the highest plant angle possible, that might be one of the causes.
Not jumping when she takes off and letting the pole take her off the ground wastes a lot of energy, thus moving the pole slower to vertical and will cause her to stall.
Or she might not be fluent with her phases. I used to have this problem in high school where I would have a decent takeoff, decent swing, but I wouldn't collapse my bottom arm enough and inside of the pole so I couldn't get my hips up and my body in line with the pole. My vault would slow up a ton and I'd fall off the pole.
*i'm pretty sure the points i just made are fairly accurate, but please correct me if i'm wrong on anything