Testing download links
You can test any buyer's actual download link(s) yourself from your E-junkie Transaction Log. Just go to Seller Admin > Transaction Log (or Free Downloads Log for free links you've sent manually or via our Updates service), then click on the buyer's Transaction ID or the Item Name of any digital item in that order (note that if you have a Common Thank-you Page URL set up, the Transaction ID link would redirect to that). This will bring up the thank-you page we generated for that buyer, where you can test their download links. Note that your own test would count against the number of download Attempts you permit on that link. If you have no problems using the buyer's download link, that would rule out any problem at our end.
Web-based downloads directly to mobile devices (e.g., smartphones or tablets, including iPads/iPhones) can be a bit of a mixed bag, depending on the particular device and what software the buyer has installed on it -- i.e., there's no way we can force a device to download something from us if it has no such download capability built-in or installed. Buyers should at least be able to download their file to a regular computer and then sync the file to their device from there. See this page for a more detailed discussion of this topic.
The standard Safari browser on Apple iOS devices in particular will only download a file if the device already has an app installed that can open that type of file -- e.g., using Safari to download a PDF to an iPad or iPhone won't work unless the buyer has already installed a PDF reader app such as iBooks or Adobe Reader.
Some nonstandard third-party software can interfere with our download process. These problems usually involve download accelerator/manager software (typically provided as a browser toolbar or extension), or some types of antivirus software, particularly if it includes a "link scanner" or other real-time Web browsing protection features.
You can ask affected buyers to temporarily disable such software if they're using any, or try using a different browser app which may play nicer with that software, and then try their download again. If you already scanned the file as virus-free before uploading, you may wish to reassure them of that, and you can recommend that they re-enable their antivirus scanner once the download is complete and, if they wish, scan the downloaded file for viruses before opening it.
Download connection issues
First, bear in mind that any download can only be as fast and reliable as the slowest, flakiest link in the chain, which is usually the buyer's own computer and ISP connection.
While broadband connection speeds are rated in bps (small-b bits per second), computers typically report download progress in Bps (big-B Bytes per second). At 8 bits per Byte, this means the maximum download rate a buyer sees would be 1/8th their maximum rated connection speed, likely a bit less due to networking overhead, so more like 1/10th in real-world conditions -- e.g., a typical 1.5 Mbps consumer-grade broadband connection could expect to see download transfer rates around 150-180 kBps at best.
There may also be temporary issues with the buyer's ISP or a network routing issue ("Internet traffic jam") anywhere along the route across the Internet between our download server and the buyer's ISP, or their computer could have malware such as a virus that's hogging all the available bandwidth on their connection or causing their computer to misbehave, or other problems may affect the behavior of their computer or connection.
Many consumer-grade ISP connections advertised as "high speed" only provide their maximum rated speed under ideal conditions and/or for brief bursts at the very start of any download connection. This initial-burst speed is suitable to accelerate routine Web browsing, as most Web pages and the images and other content embedded in them are generally rather small files, but larger files would only see that max speed as the download begins, which quickly gets throttled down to a much slower speed as the download size exceeds anything that would normally be viewed as part of a Web page. Most "bandwidth test" sites only test transfer rates using a fairly small file that would be unlikely to encounter the throttled-down speeds that some ISPs may impose on longer-term downloads in progress.
Download server performance
We use Amazon S3 to store and serve downloads on a long-term basis, which is about as high-performance and robust a download service as one can get at the current state of the art. All files uploaded to our service are now being sent directly to S3, from where we can start serving the file to your buyers immediately.
If your product uses a Remote Product File URL (rather than having its file uploaded to us), then after the first download of a remotely-hosted file stored on your server, within 15 minutes we would have a copy of that file synced up to our Amazon S3 provisions, from where all subsequent downloads of that file would be served, as long as we detect the original file has not changed since its last download. However, downloads within that initial 15 minute window would be served from our Tucson datacenter, which may result in less than ideal download speeds, as we need to throttle the bandwidth allocated to such downloads in order to keep the rest of our service available to the Internet at large -- i.e., we can't allow these early downloads to saturate all our servers' available bandwidth, as that would make our site, Admin panels, and Cart services unresponsive.