You mentioned you are using Comcast. Any chance you have a Technicolor TC8305C gateway? If so it may be a UPnP error. Comcast recently pushed new firmware to the modem and it killed parts of IGMP through the gateway LAN ports. If you check via HDHomeRun setup, you will get a UPnP error if this is the cause. Oddly, if you check Windows explorer you will not see the tuners under Network but once you run setup, after a few seconds, the tuners show up on the Network in Windows explorer.
I have 2 WMC laptops connected to the Prime. One is isolated thru an unmanaged switch, the other was connected thru gateway LAN port for convenience due to location of the laptop in relation to the Prime. The one on the LAN port began having No Tuner issues the moment the new gateway firmware was pushed. HDHomerun setup threw the UPnP error with link to page explaining why. Tried messing with gateway settings, network adapter settings, but nothing worked. The laptop isolated via the switch worked fine. So I got another switch and put it in line before the LAN port to keep the Prime isolated thru the switches and both WMC laptops work fine again. Note this was occurring with both the October and November 7 betas.