Every add-on domain, after addition to cPanel requires an Apache restart to start working. On all of the HelioHost servers Apache automatically restarts once every 2 hours. Hence, it's possible that Apache is about to restart after you add your domain, so it starts working immediately. However, there's an equal chance that Apache has restarted just sometime before you added your domain, so you've to wait another 2 hours before your domain starts working. Simply, speaking you need to wait upto two hours for your domain to start working. Do remember that you've to clear your cache at regular intervals, cuz the domain queued page gets cached into your browser and may show up even after the domain has started to work. Hope that helps.