I have taught a few people morse code and coached them for the exams in the past, but my brother made a set of tapes in the 70s and sold hundreds of them (in Australia). His approach was to divide the alphabet into groups of letters - in straight alphabetic order - and teach by repetition, in those groups. You wound the tape back to repeat the lesson. Lots of people learned successfully using those tapes. The principle was
Phase 1. learning what the letters sound like
a. hear the letter spoken
b. hear the sound sent several times.
Phase 2, trying to copy randomly chosen letters from that group, sent with wide spacing. You hear an unknown letter, followed by a pause where you try to identify it and write it down, then hear the spoken letter. This either confirms your decode, or contradicts it.
Repeat the group several times. When you have learned the first group (about 6 letters) well enough that you are confident of them, move on to the next group.
Koch method teaches simpler letters before complex letters. I believe that may work for some learners and not others.
The biggest problem IMHO is trying to go too fast when learning the alphabet, ie. moving on to new letter groups before existing ones have been learned fully. Then later the problem is not being prepared to listen to a speed you can’t copy. it is the only way to speed up, by listening even if you are only copying 50%. Tomorrow it will be 60%, etc. The same applies to learning how to send faster.
For some people the biggest problem is getting a key and practicing before they actually can receive reliably. But getting on the air and having some contacts, albeit painful, sometimes helps people.
But everyone is different and finding how each person learns is the “key” to success.
While it is accepted wisdom that listening to actual contacts helps, about half the morse I hear on the air is very poorly spaced and does not encourage learners because it is so poor.
Andrew vk1da & vk2uh