Change the word tuning to technical “placing”There are many reasons why a student has this problem.E.N.T Ear nose and throat,any of these factors could in fact play a part.So I trust you have the ability to teach place through pure air stream,using headvoice and sinus structure.
The bottom line is you can guide but you yourself as a tutor cannot complete the job for the student.They themselves must be able through a placing, when the sound is correct.In time this works.The methodology is actually vibration.