If no bulge - push her forward into taking large steps (at trot doing a 20 meter circle), ask her for long and low AND make certain her nose is on, or slightly in front of the vertical. Make certain the contact is staying consistent by opening (as you rise in post) and closing (as you sit in post) your elbows. Rider can only look down with eyes NOT head - look straight ahead (not down) and post straight up and down.
Idea is horse should be carrying herself using her back muscles. Bulge in neck indicates she is doing so, no bulge probably means she's falling on her forehand.
Once she does this consistently then keeping the trot shorten the reins for a circle then back to long and low. Let me know if you have issues when you shorten your reins.