Try one method and if that doesn't work, try another and another until one "clicks" if one method is not working, try another! And I am not talking about just Koch and Farnsworth. I am talking about E, I, S, H, 5 versus A, B, C... and 1, 2, 3... versus character speed versus character spacing versus straight code QSOs versus games. Try anything and everything! I can almost guarantee that if the student is willing to put out the effort, something out there will click with them and they will learn Morse Code. There is a reason people have made so many different "paths" for folks to learn Morse Code.
I think the same can be said learning almost anything. I wish more teachers that I had in school would've had the patience to try different learning paths like you're talking about--a lot of students would do much better, maybe even get encouraged to go further in college!