I noticed that some of the book cover images from Amazon are not appearing in some records.

Amazon book covers only appear if the ISBN in your Koha record matches the ISBN of the item on Amazon.com. It's very common for there to be no cover on Amazon, especially for non-book items like CDs or DVDs. If you post an example we can know for sure that the cause is. -- Owen -- Web Developer Athens County Public Libraries https://www.myacpl.org