I was struggling with this, my app was installing but not complete (almost 60% I can say) in iOS8, but in iOS7.1 it was working as expected. The error message popped was:
"Cannot install at this time".
Finally Zillan's link helped me to get apple documentation. So, check:
ax.init.itunes.apple.com
is not getting blocked by your firewall/proxy (Just type this address in safari, a blank page must load). As soon as I changed the proxy it installed completely. Hope it will help someone.