You can test a buyer's download link yourself. Go to Seller Admin > Transaction Log, find that buyer's transaction, and click on their Transaction ID. That will bring up the very same thank-you page with download link that the buyer was using. If you don't have any problems completing a download from their link, then any issues would be at the buyer's end or otherwise outside our control -- e.g., issues with the buyer's ISP or network routing glitches somewhere along the path across the Internet between Amazon S3 (which we use to store and serve downloads long-term) and the buyer's ISP. Note that your own test would count as an Attempt against the link, so you may want to use your Seller Admin > "Re-activate expired links" feature to reset their link expiration.
It appears most of your buyers aren't even using up all the Attempts you permit for each product, so maybe they're running out of Hours? A permanently-stalled download is usually caused by the product's expiration settings being set too low, especially when the buyer's connection is really flaky and requires them to keep resuming a stalled download (their browser or download manager may do this automatically), which uses up all their Attempts -- once all the Attempts are used up or the Hours expire, then a stalled download cannot be resumed. These help pages go into more detail about that and other common issues with expired links and download performance:
If you do get a report of a stalled download that you can reproduce by testing the link yourself (and the link isn't expired), please email our Support team with the affected Transaction ID, so we can have our Developers research what exactly may have happened there: