I knew a classical flautist, years ago, who couldn't get their head around the idea of composing their own music and, in particular, improvisation. She was quite excellent at her instrument whereas, in the band I was in at the time, writing our own stuff and improvising were completely natural to us, it seemed like the kind of thing every musician would just instinctively know and do, but not so.
When I'm writing music, I usually hear everything, all the instruments and melodies. The process of making a real song out of it frequently becomes a tedious chore, as taking it out of my head and into the real world often involves lots of compromise because my ability to play the required instrumentation and manipulate my voice, in particular, frequently doesn't match what I think the music requires. I have to work hard to get rid of the pre-conceived idea and work at something I can actually do and record in real life.