I can think of a few, first in formost, there is 2 different type of struggles
transcribing in real time which requires you to hear and identify chords and melody
and improvising, which requires you to be able to know and hear what notes you want to play before you play it.
I think most people either can do this but can't do that or can do that but can't do this
but another problem is that beginners like me cannot even identify scale degrees/function in the context of music, heck I'm having trouble identifying or singing 2 intervals in a row.
here is my thought process from this example: sing a major interval from C then sing a major interval from E
I first play C then sing up a third no problem, then I repeat the note E and sing a major third to G#? what's the problem with that?
I just changed keys, basically, I didn't hear if as a function functioning in the key but rather changed the key in my mind in order to sing that major third.
anyways learning ear training is super hard for me. my end goal is to be able to transcribe songs in real time and compose but I don't know what I'm really suppose to know
then there's out of key notes like modes