IDevices Periodically Lose Connection to iTunes Server

Question

Periodically, our Apple TVs and iOS devices seem to lose "knowledge" of our iTunes server.  This has worked fairly reliably in the past, but seems more problematic now (I'm not sure if this is due to Yosemite or some other update).  There have been no changes in our network.
I am a senior-level IT systems engineer.  Here is a relevant description of our simple home network:
Network:
Time Capsule 3TB:  Main router on upper floor
iMac 5k (connected via ethernet, though Wi-fi is also enabled as a secondary connection)
DHCP with manual IP address so it doesn't change every 24 hours (set on both Ethernet and Wi-fi)
Set to wake for network access
Firewall enabled, but iTunes connections are allowed (tried disabling firewall entirely to no benefit)
iTunes always running.
External USB drive for iTunes library storage; this does take a bit to spin up and sometimes the VNC connection to the iMac can timeout when trying to connect, but it always works on the second try.  I wish Apple would increase that timeout; seems to be a measly 5 seconds or so.  I don't think this is a cause of problems because, for example, the iOS devices don't even see the "Shared" icon, the display of which does not wake up the iMac.
Airport Express (N version), lower floor, connected via ethernet to the time capsule.
This is used as a network extender, operating in bridge mode with no network filtering.
Apple TV (3rd generation), connected to Airport Express via ethernet
When Home Sharing is working, the Apple TV and iOS devices can all connect just fine to the iTunes library running on the iMac.  This is true even when the iMac goes to sleep after 15 minutes.  It can work for several hours, but at some point the devices simply lose sight of the iTunes library on the iMac.  This is not supposed to happen because of an Apple service called Bonjour Sleep Proxy.  This runs on Apple TV and Time Capsule/Airport devices, and it wakes up the server you are requesting.  I thought perhaps the IP lease was expiring on the iMac, but setting a static IP didn't help.
I have more issues with iOS devices than I do with Apple TV.  Quitting and re-launching iTunes doesn't seem to help iOS devices.  EITHER restarting the iMac OR restarting the Time Capsule will resolve the issue (causing the Shared button to show up on the iOS device) temporarily, but it's guaranteed to happen again a day later.
I haven't tested setting the iMac to never sleep, but I will.  This is not a long-term solution, though-- I'm not going to leave a $4k iMac running non-stop 24 x 7, sucking in all the dust it can.  I also haven't tested putting all devices on the same subnet, but this shouldn't be needed-- Bonjour Sleep Proxy will even work over the internet, around the world.  Unfortunately I can't easily test that, as the Apple TV/Airport Express is downstairs and the iMac/TimeCapsule are upstairs.  However, it doesn't matter whether the iOS devices are connected downstairs or upstairs.  When it's working it works on both WiFi basestations, and when it doesn't work it doesn't work on both.
Any ideas?

Answer 1

A small number of users have reported having to keep their Macs from sleeping. Can't say why and most of us don't have to so it does seem to be something specific to set up.