In Django admin, why are some foreign keys showing the green plus icon, but others aren't?!

I've been working on a model that had several foreign keys, like this:

class CaseStudy(models.Model):
    company = models.ForeignKey(
        Company,
        on_delete = models.SET_NULL,
        null=True,
        blank=True
    )
    quote = models.ForeignKey(
        Quote, 
        on_delete = models.SET_NULL,
        null=True,
        blank=True,
    )    

Problem was, in the admin, I could NOT figure out how get the Quote field to have the green plus icon next to it. It worked just fine on the company:

But nothing on the quote:

The answer was to register the quote model in my admin.py file:

admin.site.register(Quote)

I had already done that for the Company model, but I had kept Quote buried as an inline on another model, and didn't see the need to list it in the Django admin. Now that both the Company and the Quote models were registered to the admin, the add, change, and delete buttons have become available to my foreign key fields.