Thanks, Drew! I can probably post a video.
It’s definitely the vertical punch. It might also be that I’ve gotten too ambitious and skipped steps… I mean, I don’t even know punching bags or any proper looping. Maybe trying for StM was biting off more than I can chew (mind you, I did that for 1A also, going for Spirit Bomb after only 2 or 3 other easier tricks like Matrix and Kwyjibo).
For the vertical punch, I indeed aim to go a little back, and I’m actually trying to go gently. The few times I’ve “almost” gotten it, I could feel how it’s supposed to work. Which is a good step. But when I try to reproduce, I’m not adding any more consistency. 
Typically, as soon as the upswing starts, the yoyo goes out of alignment… flattens out horizontal most of the time, or at least gets all janky.