I've been trying to resolve this problem for weeks now. After a crap nights sleep I was sat up looking round the Apple forums when I stumbled across the below thread.
They traced it o the ATV 3 not releasing ports and stopping iTunes from streaming. Simple fix is to take any ATV3 which are on LAN and put them on wi-fi. Nothing more, you don't need to keep them out of sleep mode or fix ips, or mess about with your routers settings.
My iTunes was locked again earlier, I pulled the my ATV3 off LAN and turned wifi on, now without even restarting iTunes on my mac I was able to load all the sharing libraries straight away.
Hope this helps
https://discussions.apple.com/message/18378253#18378253