Why must I use the Kindle trim for banners?
The banner templates are built around the proportions of the Kindle front-cover render. Print trims include different proportions and bleed, so they are not offered as banner source images.
If your latest cover changes only exist in another trim, return to the front-cover editor, select Kindle, apply the same changes, and click Remake Cover. The new Kindle render will then be available for banners.