It may depend on your download speeds. Large files can, "time out" before the download completing.
Check your download speed here > Speedtest.net - The Global Broadband Speed Test
And from the App Store menu bar top of your screen click Store > Check for Unfinished Downloads
The amount of time it takes to download your purchase will vary according to the file's size and your Internet connection speed.
Just as an example, check out the information for the iTunes Store > iTunes Store: Download times will vary
It's not an Apple issue. Contact your ISP. Maybe you can upgrade your service.