Some students be from any part of the world just don't get it.
And what ever the instructor does they won't get to a suitable professional level.
If the student won't do the ground school and put effort in there isn't much the instructor can do about it. Learning to fly is hard work and requires commitment.
Instructing is a two way thing. And both parties need to be moving towards the same goal.
And the instructor doesn't fail if a student doesn't come up to scratch. They bank their wages and move on to the next one in the knowledge that someone that couldn't make the grade isn't being let loose in the sky.