I have the exact same problem. My apple tv asks me to turn on home sharing even though it is already activated. After going through the list of trouble shooting items, I am still not aware of the reason. Thanks!
This is possibly happening because the Apple TV does not detect another device on the network using the same home sharing details. Have you set up home sharing both on the Apple TV and in iTunes on your computer. If you have you may have a network issue such as a firewall blocking access to iTunes.