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For more information on this, take a look at this article:
Apple TV (2nd and 3rd generation): Troubleshooting Wi-Fi networks and connections
What you can do if, you cannot access your shared iTunes library over your home network
Troubleshooting Home Sharing
Best of luck,
This will most likely help a bunch.....it did a complete fix for me. Follow it step-by-step.
1. Open "System Preferences". ( computer only), (Windows computer unknown)
2. Open "Network".
3. Click on the "Sprocket with down arrow".
4. Select "Make Service Order"
5. Move the connection your using (Wifi, Ethernet, etc.) to the top, and click Apply.
6. If you selected Ethernet, then turn OFF Wifi in the top menu.
7. Shut DOWN your computer and then restart after completely shutting down.
7a. Either restart or unplug and plug in the Apple TV (insure your logged into Home Share after restart).
7b. Start iTunes (insure your logged into Home Share).
Enjoy life again!