If it doesn't click, it's probably that it's just harder (for most people, I guess). Theory is not done in real time. It's visual and you can think about it as you do it. Ear training is in the moment and calls into play certain aural skills that one doesn't normally use. What worked for me? Don't know because it was an accumulation of things. Here's the chronology. Sight singing in high school. Playing tunes by ear (also in HS). Sight-singing and ear training, systematically in college and with added concentration on chord progressions. More playing by ear, this time including playing along with recordings. Gradual eye-ear association by conducting and also transcribing. Eventually, I got to a point where I could sight-sing and take dictation of atonal music. Sorry, I can't break it down for you.