One possible reason your having problems downloading is that your internet connection has issues or just needs reconnecting, reboot your device and make sure the wifi is connected, maybe even try a random search on google
Another possible issue is that your using a fake or faulty SD card, so when you 'think' you have plenty of space for the game, in reality you dont. To check if its potentially a fake, you can open Settings, goto system, system information and check your Capacity (NOT free space) with the chart on THIS page. Another way is to simply run the 'Quick App Dowloader' to see if it shows a warning
Another common issue is that you possibly have partial (possibly corrupted) downloads that need clearing.
Open vitashell, goto ux0:pkgj/
scroll down and look for any files with '.resume' and delete them
goto ux0:bgld/ and delete its contents