We have received reports that on some iOS devices, after associating to the WiFi signal the captive network assistant can take up to a minute to trigger and display the splash page. This is behavior of the iOS device and not related to the Marketing4WiFi platform. The issue was originally reported to Apple on 2/12/2019.
Apple developer forums article
From testing in our lab a possible solutions to iOS devices exhibiting this behavior are to perform a Force Restart on the device or a Reset Network Settings.