iOS 8 CNA not triggering
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.