When a teacher of God fails to heal, it is because he has forgotten Who he is.  Another's sickness thus becomes his own.  In allowing this to happen, he has identified with another's ego, and has thus confused him with a body.  In so doing, he has refused to accept the Atonement for himself, and can hardly offer it to his brother in Christ's Name.  He will, in fact, be unable to recognize his brother at all, for his Father did not create bodies, and so he is seeing in his brother only the unreal.  Mistakes do not correct mistakes, and distorted perception does not heal.  Step back now, teacher of God.  You have been wrong.  Lead not the way, for you have lost it.  Turn quickly to your Teacher, and let yourself be healed.

ACIM. A Manual for Teachers. 22.5