Yun not showing in IDE (when other Yuns Do)

I have 4 Yuns in my house controlling LEDs. 2 originals and 2 brand new REV2s.
All four are configured the same way in my router with static IPs. All have unique names on the network, ArduinoX (1 - 4). Three of them show up in the IDE (1.8.7) one does not. The odd one is accessible via IP, and is running my sketch and talking to the other Yuns as it should (they sync modes). Everything seems totally fine with it, except it won't show up in the IDE which makes sketch updates annoying.

By "it won't show up in the IDE", do you mean that no port appears in the Arduino IDE's Tools > Port menu when you plug it into the computer using the USB cable?

If it's plugged in via cable it shows up. I does not show up under the network ports with the other 3.

Do they all have the same firmware ?
Do they all use the same network settings (sounds like you may have a setting incorrect) ?

The two pairs are on different firmware, old pair and new pair. But each pair is on the same.
As far as I can tell they all have the same network settings on the device, and they are all definitely the same on my router.

Some (not all) routers group IP's.
EG. all Lan might be 192.168.0.2 to 100 and WiFi into 192.168.0.101 to 200 etc (example only)
Might be worth checking for.

Some Routers also accept or change a connection to IPV6 over IPV4.
I have some devices that show under both protocol.
No I dont understand why they work that way but they seem happy until I try call them up by an IP at which point I occasionally run into issues.
Just something else to keep an eye on.

All 4 devices are set to always receive the same IPv4 IP. So it's not an IPv6 issue or an odd IP.