Simple Control’s automatic discovery relies on various fundamental networking technologies collectively known as ‘multicast’. This has been used since the dawn of networking for simple things like finding printers and is now useful even for discovering your A/V equipment.
Some network routers are not configured correctly to handle multicast. In particular, any option labelled “Enable Multicast” or “Enable UPnP” should be turned on. Those are the most common causes of automatic discovery problems.
Some routers just have bugs as well and don’t handle basic networking tasks properly. These may vary from time to time with firmware updates. One in particular that is a known problem is the Actiontec series of routers often used with FiOS. The solution with that router is to disable the IGMP Proxy setting. If you are using a revision 1 Actiontec with the latest firmware and do not have the IGMP Proxy link, log in to your router then go to http://192.168.1.1/index.cgi?active_page=6059. If you have changed the IP address of your router, make sure to adjust the URL accordingly.
If you’re still having discovery issues after checking those settings, make sure to restart all involved devices including the router, iOS device(s), and target device(s).