Actual cover image not showing in Calibre. Shows blank white image instead

[GalacticHull]

The subject basically explains the problem. Certain titles, largely ebooks published over a decade ago, show a blank white image in the cover browser. I've attached a couple screen shots to demonstrate the issue.

I'm sure this is a problem with the epub itself and not calibre.

If anyone is aware of what I can do to tweak the epub itself--either by hand or using Sigil, and certainly not using Calibre's editor--rather than just adding the cover in calibre, I'd appreciate it if you'd fill me in.

[GalacticHull]

note: Think I figured it out, though I'm not certain why it works.

If I remove

<p><a id="cover"></a></p>

from the cover page, it appears fine.

That said, there is a cover id in the manifest. Despite this, Sigil validation doesn't present an error when removed. Perhaps it's because Cover is not in the TOC? I'm very uncertain.