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A frequent cause of consistent failure to enable AirPlay or HomeSharing at all, is the service being blocked on the network. Make sure your network isn't hidden, has a unique name, that MAC address authentication is disabled, security is set to use WPA 2 Personal and that there is only one router/device acting as a DHCP server and providing NAT services.
Make sure your router/computer allows access over the following ports
Refer to your router manual/manufacturer for any settings that are specific to that model.
Another frequent cause of consistent failure to enable AirPlay or HomeSharing at all, is security software, in many cases configuring it correctly, disabling it or even uninstalling it can help, but in some cases the security software can cause problems that simply reconfiguring, disabling or uninstalling cannot reverse.
If you are consistently unable to activate AirPlay, have tried all the steps in this article and have security software installed on your system, you might benefit from contacting its provider or participating in any online forums they run to discuss the matter with them.