I know I’m really late to this party, but I thought I would share my experience since I am still really new, too. I have been playing since March and taking formal lessons since the end of March. As others have said, 3, 4, and 6 are pretty well programmed in, not just to hit them right most of the time, but to hear and know when I hit them wrong. 9 is mostly there, but I still miss occasionally. Everything else is rocky.
I started learning Ichi-Dan a few weeks ago and it has a long-ish run from 3 to 19 (3, 4, 6, 9, 10, 12, 14, and 19, if if remember correctly). I tend to miss 9 fairly often if I am trying to go too fast. And as someone else said above, the spacing gets kind of weird past 10 or so and I am really struggling with hitting them all consistently.
But, as time goes on, it’s starting to get easier. One thing that has helped me is just to sit with my eyes closed and play. No sheet music. No finger markers. Just listen and play. When I hit the note wrong, I’ll open my eyes and adjust, then close them again and play. Besides being almost meditative, it has really forced me to develop some muscle memory.
I think what @ChristopherBrown (who is far more skilled that I will probably ever be) said really dawned on me about a month ago. A good snap really is more about technique. It’s not about strength or velocity. You can actually do it very slowly and deliberately if you try. Think of it as trying to trap the string under the bachi in just the right way so that it pops out and snaps. That helped me at least.
Out of curiosity, what kind of bachi are you using? I could never get a good snap with a plastic bachi. Maybe I can now, but I haven’t tried since I ditched mine. I hated that thing…