Built-In Hotspot

Can someone fill in the gaps on how the built-in splash page works? I have a couple networks with a guest wifi and using the internal hotspot. I get reports that users aren’t getting the splash page when they connect and therefore can not use the WiFi.
As soon as I try and connect I get it right away every time. Is it something with their devices? I don’t really have any filtering configured as far as firewall goes. One setup is just a single vlan with a SOHO router, and the other one is actually a separate vlan, but I’m doing all the vlan work on a 3rd party router and there is internet access.
Just makes no sense that anytime I try it works no problem, so it’s really difficult to troubleshoot.