I think STG mindset is a very special one, and one you must "get" into to have any progress. You won't do that by just giving the games any number of runs a day, it's not something that is guaranteed to come if you try playing aimlessly, unlike some other genres.
To clear up a bit, myself as example. Before I took an interest in Touhou, I did play STGs casually on occasion. Even when I had long sessions with multiple runs in a row, there was no sign of progress in sight. That's because my aim was to just enter there and shoot things. My tactics were no more than "bomb till out of bombs, stay still and hope to not get hit afterwards". I had no idea there was any other way to play, and I didn't need one. But then, it just... came. A craving for seeing many bullets fired at you, and dodging them all. It came when I mastered a certain FPS, and it wasn't your generic "interactive shooting range" or "hide behind corners to not die" stuff like CoD - it was the legendary Doom II. You know, the one where there are mostly fireballs thrown at you? I've learned to dodge them like a pro. And wished to do some more impressive dodging. So, you see, if it wasn't for that, I'd still be just throwing bombs and watching fancy effects no matter how many runs in any STGs I'd do. (And know what? Turns out being a master of a hardcore FPS just barely prepared me for challenge that is Touhou on Easy.)
So, what I'm saying is no, not everyone can pick up the genre and start getting better at it. Things you should pay attention to, skills you should work on - they aren't readily obvious, you can't just pick them up as you go, unlike with most other genres. Even if you'd try to explain stuff to someone who has no idea what to do, you might just end up confusing them with things that are alien to their gaming perception (like hitbox shenanigans, that's one huge jump, I tell ya). It's not like they're mentally incapable of that, maybe it's not like they don't want to understand even. It's just that picking up some ideas and adopting certain ways of thinking may be too hard for those who've never thought in those dimensions.
However, that's not to say that one can tell who can and who can't take up the genre. STG mindset is a thing that is hard to put into words fully. Basically, it either clicks or not. Maybe it will in future. Maybe it never will. Best way to know is to try. Watching replays and videos also helps. Learning to recognize patterns is a good sign. Sometimes you just need some time after first exposure to get used too. However, if someone just doesn't want to adopt that mindset, there's no helping it. No amount of effort can do a thing without just the right kind of motivation.
Hope this all wasn't so confusing.
(Also, PCB must be the HARDEST game on Easy - it still gives me trouble to this day. IN is way better for a start. Actually, IN Easy 1cc requires the least effort to accomplish, from my experience.)