I would definitely try resetting your computer to see if that makes the changes take effect.
Your problem sounds similar to the one I'm having. From the poking around I've done, I think this is because the Mac registers itself with a device capable of acting as a Bonjour Sleep Proxy (this could be an Apple networking device, or even an ATV2/3) as being available for wake-on-network access. The Bonjour Sleep Proxy then advertises this Mac's services (iTunes Home Sharing being one of them) on its behalf. Your ATV library, other machines running iTunes and iOS devices see the sleeping Mac's library being broadcast as available. They do not know it is on a sleeping Mac, but just see it as available. They attempt to access the library, and in doing so contact the Sleep Proxy with a request effectively saying 'I want to access this iTunes library'.
The sleep proxy, however, knows that the Mac is asleep. It then broadcasts the so-called 'magic packet', which is a piece of information sent out over the network, and which a sleeping Mac (with wake-on-network turned on) is listening for. In theory, the Mac should 'hear' this piece of information, interpret it as an instruction to wake up, and then wake up and take over it's Bonjour services, in this example serving up the media content over iTunes Home Sharing.
What I think is happening, in the case where the library is seen as available by client devices but nonetheless inaccessible, is that the sleeping Mac is either not receiving or not responding to the magic packet. The Sleep Proxy is still broadcasting the iTunes library as available, on behalf of the sleeping Mac, but is unable to wake it up.
Another widely reported problem, one which I am also experiencing, is that whereby the library is not listed at all. A Mac running wake-for-network is supposed to periodically awaken to renew its registrations with the Bonjour Sleep Proxy. This isn't happening, and so the Sleep Proxy doesn't even consider the Mac as available, and hence the iTunes library is not listed as available (or even existent) to other devices on the network.
I suspect that both of these problems are a symptom of the same underlying bug. Once my Mac goes to sleep, it does not properly listen for wake-up requests and nor does it maintain its sleep proxy registrations; it would seem as though sleeping my Mac also switches off the low-level networking functionality required to perform each of these tasks. It's as though I want it to enter sleep, but it is instead entering a coma. Only manual intervention (me wiggling the mouse) is able to wake it up.
The strangest thing of all is that it isn't a consistent problem. Whilst most of the time, wake-for-network does not work, it occasionally (and seemingly randomly) does. When I'm at work, for example, and using my Macbook, my iMac at home will sporadically appear as available over Back to my Mac (which depends upon properly functioning sleep proxy registrations). When this happens and I try to access it, I'd say that about half of the time the Mac actually wakes up and allows me to connect to it.
Bizarre and frustrating.
