In order to learn to sing the person studying needs to grapple with the process in a large way. Some people have trouble doing that. They can’t connect the dots.
I have encountered this several times over the decades. The person seems only able to hang on to what happens in the lesson during the lesson. They are unable to go home and practice, keeping current skills while adding new ones. It’s as if they forget as fast as they learn.
Being able to relate one thing to another and make a “personal grid” for vocal training is necessary if the technical work is to be meaningful. If all the student does is run through the exercises, like a robot, then any exercises that aren’t flat out ridiculous, presented by anyone in any way, would be the same. It’s as if they view learning to sing in a literal way like going to the gym and running through the machines there. But at the gym, they tell you which machines do what. Then, if you just work it, it will do its job and you will get more muscle tone. Singing works that way too, but it doesn’t make the singing conscious or deliberate and your singing will be as mindless as your  use of some musical patterns on vowels.
Even talented, professional people can’t grasp the system as a system. They cannot relate one mechanical function or response to another. They are willing to be guided around, they are willing for the teacher to “fix” them in a lesson, they are willing to be “corrected” and even to record the session, but if you ask them to warm up on their own with some semblance of order, they can’t. The either can’t or won’t try to understand how things relate to each other, or how the voice works overall.
I’m not talking about beginners here. Obviously a beginner is going to be confused for a while. I’m talking about people who are working professionals with decent ability and a reasonable amount of time (a couple of years, minimum) of more or less consistent training. Couple this with an explanation given by the teacher of what the exercise is for, what it is doing and why, and you would think the mind of the student could track things more cohesively, but it’s not so uncommon for the student to be unable to do that well.
If you have a student that calls forth from you over and over again the same correction of the same problem, and you can correct it in the lesson but they cannot correct it on their own away from your studio, you have encountered someone who can’t connect the dots. Worse, even if you connect the dots for them in the lesson, often, when they leave, they don’t remember what you’ve said, even if they have recorded it.
This kind of person, after a while, needs to be confronted with becoming an adult in terms of taking responsibility for his or her own vocal capacities. Some people just plain resist that step and spent their whole entire professional life going from teacher to teacher looking for the one that will give them the “magic bullet”. That is easier for them to do than actually figuring things out for themselves, step by step, over time.
If you have a student like this, and you have worked with them consistently for at least two years, and they keep going over the same information with you, either you have to confront this with them or you will end up being frustrated. Don’t waste your time on people who refuse to integrate what you are teaching.