1. They are not supporting the tone enough, and as soon as they develop firm support, they are able to hit their notes on pitch!
2. Going along with this process, I work on their limited belief system.
Somewhere early on, most of these people were TOLD that they were 'tone
deaf,' and so that became their 'story.' Into adulthood, they still cling to that belief until through gentle awareness and positive affirmations, they can let go of that and start to believe that they really can sing on pitch!
Now there are students who really have a challenge in hearing and singing the correct tone. I've been working with one gal for about a year, and she has come from singing the melody a third low consistently, to singing wonderfully on pitch. Again, the process was a combination of physical technique (involving the body; releasing tension; support etc.) along with visualization; thinking down not up; gaining confidence in her abilities.
Every student is different and as you 'peel the onion' you discover what works best for that person.