I have twice encountered this issue with an aggregator named PublishDrive. On the first occasion it was with an EPUB file that had nothing to do with Pubcoder - I'd put it together by myself and had gone through agonies getting it validated. When it was finally validated, PublishDrive still rejected it, the reason being that they could not find or identify the cover. I read all their documentation about this and tried every solution, none of which worked. I reached out to their support people and one of them went in and manually fixed the problem at their end. I don't know what they did, but it worked.
I've just encountered the same issue with PublishDrive, and this time it's with an EPUB created by Pubcoder. I know the cover's there and specified, so I've written to PublishDrive again for help in solving the issue. It does seem to be a peculiarity of that site. I'll update here when they reply.