I'd say its a combination of all three, but Fields is the smallest one.
Over the past three years, outside of Mac Jones, Fields is the one who got the worst coaching altogether (Urban Meyer > Matt Patricia, I don't make the rules). The Bears are a ****show with no direction. It never felt like the offense was geared towards Fields, and it still doesn't.
The few times Chicago constructed a gameplan for him, he balled out. I remember my Patriots playing against them last year on MNF expecting our defense to perform a dominant performance and he hung 33 on our ***.