I can hear chords. I can hear two-part harmony. Yet my aural imagination wasn't developed enough to generate novel music, except when I was in certain moods or about to fall asleep. And most of what I could hear in my head I found impossible to transcribe.

I had exactly the same problem - and with the same strange exception: Music sometimes start to flow through my head just before I fall asleep, in multipart harmony.

I'd be interested in any ideas as to what makes this mental state different, and how to cultivate it.

I find it's easier to imagine novel music if I restrict myself to a particular genre+style. Just settling on the instruments helps a lot.