It sounds like she is getting lazy and she thinks she doesn't need to tuck as much because she knows she can clear it without really tucking.
You need to do a lot of jumping work where the obstacels are spaced close together so that she has to tuck to get over them.
Also, try putting ground poles before the jumps. This will get her feet moving and prevent her from rushing the jump which a lot of horses do so they don't have to tuck as much.
Trot them at first and give big squeezes with your legs.
Exagerate your half seat and lift her with your body.
This will get her thinking the jump is a lot higher than what it is and she will lift her legs higher.
You don't necessarily need to raise the jumps to get her to tuck, because this will get her thinking she only needs to tuck well over the high ones.
We want her to tuck over everyhting, even a tiny corss rail.
I had the same problem with my 4 year old stallion but now he tucks and flys over jumps from 2 to 4 feet the same way from doing these exercises. Hope you get her tucking like a pro! Good luck!