Some mobile devices (including iPhones) don't allow native music file downloads, so downloads won't work like they normally would on desktop or a compatible device. Fans do get an email with the track so they can download it later.
We do our absolute best to test our software with as many different browsers, operating systems and hardware combinations possible. That said, it's completely possible that your fan attempted to download a song on a combination of platforms we’ve never tested. When fans say a download (or any other tool of ours) isn't working, the best thing to do is to ask what hardware, OS and browser they were using so that we can try to recreate the issue on our end and fix it!