Download links from S3 not working for purchases

I'm having a problem downloading files that we have purchased using MarketPress.
We've done two purchases, and the download links are going out with the emails properly, but the links never go to the ZIP file downloads.

The admin page for the order shows the file has not been downloaded, either. No error messages are

The files are being hosted on Amazon S3, and have been made available for download (We saw another error when the file hadn't been made public)

I have just tried this with the file linking to another public server, with a JPG instead of the ZIP, and this worked. Both files should be accessible through the links being used.