The spine or back rounds due most likely to tight (or closed) hamstrings. The pull on the back of the pelvis preventing it from tipping forward.
For students in this situation an appropriate way to instill the feeling of Adho Mukha Svanasana would be to bend the knees (let go of the idea of rooting the heels even though that is "right"), press into the index finger and thumb mounds, and recoil the sitting bones back and up toward the top of the wall behind you.
In addition it appears that the distance between your hands is far wider than your shoulders AND the distance between your feet far wider than your sitting bones. This too can be adjusted, though it will not address the specifics you've broached in this post.
Work the hamstring opening in Supta Padangusthasana twice per day - morening and evening. Do three rounds of three breaths on each side (alternating) during each session. Ergo each sesion will consist of 9 breath on the left and 9 on the right. Use a strap and have your teacher instruct you in the three alignments and three actions of the pose.
And, of course, be patient.