White Line or Gap at the Spine After KDP Upload: What's Really Wrong

You uploaded your cover, opened the KDP print preview, and there it is: a thin sliver of white running down the spine, or a visible seam where the front cover art doesn't quite meet the back cover art. This is one of the most common reasons a cover gets bounced back for revision, and it almost always comes down to one of three root causes — not a mysterious rendering glitch. This post walks through all three, shows you why a spine error you'd never spot in Photoshop becomes obvious on a printed proof, and gives you a step-by-step way to confirm exactly what your PDF should measure before you re-upload.