Hi mate,
Are you starting those downloads simultaneously?
If you do, that's the case as QuickSell marks the download as complete once the last byte is sent to the client.
If your files are big you are able to start multiple downloads without finishing them, therefore succesful download is not being counted, because you don't want an user to download half of the file and run out of downloads in case the broadband resets the connection.
Do you think that's the case?
Looking forward to your reply
Kind regards,
Deian