This is a very common problem in teaching beginners. I have discovered that many students do not understand how to set up for the pose, and therefore the chest and arms collapse.
Try this: From Plank Pose, have the student place the knees on the floor. Now clearly demonstrate that the hips need to shift back and align directly over the knees before she attempts to lower the chest. The student should be on all fours, hips over the knees and shoulders over the wrists. If the hips are not aligned properly, but are allowed forward of the knees, there will not be enough room to place the chest between the hands. The chest will collapse and move forward of the hands, and the student will fall on her belly.
Once the hips are over the knees, have the student lower the chest. Remind her to aim the chest right between her hands. Keep in mind that some students may be weak in the arms, so it could take time to build up the strength for this move.
Maty Ezraty has been teaching and practicing yoga since 1985, and she founded the Yoga Works schools in Santa Monica, California. Since the sale of the school in 2003, she has lived in Hawaii with her husband, Chuck Miller. Both senior Ashtanga teachers, they lead workshops, teacher trainings, and retreats worldwide. For more information, visit http://www.chuckandmaty.com.