I usually have to "piano" the buttons. I play on a stick and as soon as i jump i hit 1, 2 (or 2, 1, doesn't matter which first) quickly. When forced to play on a pad, i slide my thumb over 1, 2, hitting both buttons one after the other. I typically nail the NJP the majority of the time.
I have issues hitting NJP with some chars but with f21 njp it's pretty simple. I think the tip I have is to jump when the opponent is at their peak and hit njp a fraction of a second after she leaves the ground. piano-sliding the buttons helps sometimes but i very rarely miss the njp even online just hitting 1.
Hold up once the f2 hits and punch when you leave the ground. Much easier than say, Kung Lao's or Sheeva's neutral jump punch combos where you have to move forward slightly.