Unfortunately it isn't possible to completely eliminate all possible download problems, we can only control our end of the connection and make sure it is always up and running. Since we deliver files from Amazon's servers we can promise that there won't be problems on our end, but a download can only be as good as the weakest link in the chain, which can be anywhere else along the connection... usually at the buyer's end itself where there might be old hardware or poor internet service or any other number of problems.
If someone reports a problem with a link you can test it yourself by going to Seller Admin > Transactions Log and clicking on the transaction ID for that sale, and you can also see how many times the buyer actually tried their own link. If many buyers are complaining that their links are expired before they could complete their download you might want to consider increasing the number of attempts you allow. For what you describe 3-5 attempts should be more than enough for most every case.
Fortunately most of the problems a buyer's individual connection can have also tend to be temporary, so the best solution tends to be renewing their download link or sending a new one so that they can try again after some time has passed since their first attempt.
If buyers are using mobile devices and having trouble with downloads they may need additional applications on their end to allow that, or they can download to a PC and transfer the file to the device afterward.