In the report, for Image feed, you get one or several messages like this one:
Error 20005: SKU: XXX-YYY - Cannot associate image with an existing product. For more details, see http://sellercentral.amazon.co.uk/gp/errorcode/20005
or
Error 20005: SKU: XXX-YYY - Images could not be associated with this SKU. This error may be resolved by addressing other product data errors. For more information, see: http://sellercentral-europe.amazon.com/gp/errorcode/20005
The feeds are sent sequentially but can be processed in parallel, which can result in an error in Amazon.
In Amazon, different servers will receive the feeds, and process them independently from each other.
Therefore it can happen that an image feed is processed while the product feed is still being processed.
Thus, the products present in the product feed are not yet created and the image feed is processed. So this image feed concerns products that do not yet potentially exist, explaining this error message.
The solution is to send an independent image feed for these products, by sending a feed in sending offers mode (synchronization) and selecting “Send Images”.