For those having trouble with single notes, here’s an idea. Some of the cheaper harmonicas may be a good way to learn to isolate single notes. I personally like the Johnson Blues King Harmonica. It’s hard to play, cheap in quality and price, and, yet, very often it produces the sound that I prefer. I don’t claim to be a harmonica teacher or even “accomplished” on the harmonica, itself. But, I do love ''playing" it!
It takes a lot of air to play the “Ole Johnson” and it can be difficult to get even one single note out of it. However, if someone is trying to learn how to hit single notes clearly, it might just be what is needed? It almost “isolates” the notes for us. After playing it for a while, we can become accustomed to hitting the single notes at ease. If we think about our technique as we play, we should be able to transpose this manner of selecting single notes over to a much better harmonica. Using the technique we learned on the “Ole Johnson”, hopefully, will help us to play single notes better.
Now, this just a theory, mind you. I already knew how to play single notes before I “discovered” the " Ole Johnson", a few years ago. But, I think it’s something to try for anyone who’s becoming frustrated with the single note playing.
Take care and have a good day!