iOS 8 CNA not triggering

I'm in the process of implementing a captive access point and when testing with iOS 8, I don't get shown the captive network assistant.

When I use Android and BlackBerry, they're able to detect that they're on a captive portal. It just seems like iOS which is the odd-one-out, refusing to display any kind of prompting for the user to sign into the network.

My portal is implemented by redirecting all traffic at an IP level to a local server, which then generates a genuine HTTP redirect to our portal.

iOS detects captive portals by attempting to access http://www.apple.com/library/test/success.html or http://captive.apple.com/hotspot-detect.html with the user agent 'wispr'.

Should this page request not return "Success", but still return a HTML page, then it shows the captive portal assistant. Make sure that you are correctly serving your captive portal page to this URL.