First, bear in mind that Apple only allows direct downloads to their iOS devices (iPad, iPhone, iPodTouch) from their own App Store; they do not allow direct downloads from the Web to those devices, so those buyers would need to download using a regular computer and then sync the file from there to their iOS device.
You may wish to add instructions to your thank-you/download page advising iOS buyers accordingly, and others to simply click the link to download (rather than trying to do a right-click > Save As..., which will not work). You can add this message on every thank-you page by putting it in Seller Admin > Account Preferences > Common Thank-you Page HTML, or add it to the thank-you page for each product in the Message (HTML) field of the products' settings.
We haven't had any other reports indicating a general problem with our download system lately; these help pages cover the most common reasons for slow or stalled downloads or links that seem to expire prematurely:
I notice one of your products is configured to only permit 3 Attempts at starting a download; this is quite likely to increase the proportion of buyers who are unable to complete a download successfully before their link expires due to the reasons explained at the links above. We recommend sticking with our default expiration settings allowing up to 5 Attempts within 120 Hours.
That said, only about 5% of your buyers in the past month used up all their Attempts, so they may be running into other issues like those explained at the "slow-stalled" help link above, or they're just waiting too long and trying to claim their download after the Hours limit has already been met (this counts Hours since payment, not starting from their first download Attempt). You can always reactivate expired links in your Seller Admin > "Re-activate expired links & Re-send thank-you email".
If they're not receiving your thank-you emails with a link to their download page at all, ask them to check their Spam folder and mark the message as Not Spam. If they still cannot find the message, you can go to your Transaction Log in Seller Admin, right-click on their Transaction ID and Copy Link, then paste that thank-you page link into your personal reply to the buyer, and may be a good idea to reactivate their link right then while you're at it.