Adding or changing Cover artwork with Calibre?

I've been playing around with Calibre. I'm starting to figure out things for the most part. But some files don't have cover art or I am unhappy with the current. Simply changing it, and reloading the book doesn't seem to help. But if I download and update the metadata and add a cover it has about a 50% success rate of changing on the Kindle.

Is anyone having similar problems or even recognized the variables at hand?