In those instances, it's usually a case of a teacher having a certain number of ways to describe a concept and eventually hitting upon the one that works with that student. For example, when I am trying to get a student to understand how to pull up the front of her hips, I may emphasize lengthening her back or connecting her hips to her ribs or lifting her abdominal muscles or any number of other ways. Eventually (I hope) I will get the one way that works for her.
But then there are the (admittedly rarer) instances when I have a breakthrough too. I might have a student who has a physical limitation that prevents her from applying corrections I am giving her. Or she might be applying them but the result is still not what I want, what is correct. Because my classes are relatively small, I have the opportunity to give personal feedback and individual corrections, which allows me to really dig into ongoing issues with my regular students.
Recently, I had a student who was technically advanced and diligent about applying corrections. She is intelligent and understands her body and how it moves but no matter how many different ways I would give her a correction, we could not get the right result! It was frustrating for her because she was struggling to get the right outcome and hearing me shouting about it all the time. And it was also frustrating for me because I had not gotten it. I had not figured out what the problem was exactly in order to fix it.
And then finally - finally! - I got it! I was watching her during a private lesson and I saw something that I thought I could fix. When I suggested she try something (in this case it was moving her heel across her knee during an en dedans pirouette), it worked. I was so excited about it that I asked her to apply it to a petit allegro step called brise vole - and that worked too! I cheered! I had been working with this student for months and eventually I got it.
In this instance, the correction was one I would never give to a class in general. It was so very specific to this student that it couldn't really be applied in the broader sense. But the wonderful thing for me was to know that my instinct was right. Even though it was not a traditional correction, it worked for her. I was so happy for her too because it was something I knew she could get into her head, into her body, and make it work for other steps.
Never ever say never. Never say you can't do something. Never say your body won't move that way. Never say a student is unteachable. If you take the time, trust your gut, try new things, eventually something will click. And when you both get it, cheer!