Iām in this situation. Hub Mesh was working for months. Then this week it stopped. Hubs are on the same non-managed switch, no wifi Mesh. Tried TCP it didnāt work, rebooted everything. Nothing works to get it back on.
This is a problem if there isnāt a reason it happens and canāt reproduce it is very hard to fix.
When something has been working fo a while and then stops the first thing I ask myself is "what changed?" Hub update, network change, enable hub security, network settings on hub, anything like that?
I must admit, this one caught me out. 
Iāve added two z-wave devices. No other known changes.
I did get it working.
A. Switched to TCP on both hubs - didnāt work
B. Switched back to UDP on both hubs - still didnāt work
C. Checked the network for multicast and other settings (made no changes is everything is enabled) and both hubs are on the same non-managed desktop switch.
Stumped and not wanting to remove the two new devices (they are August locks and hard to pair).
This worked:
Wellā¦ glad it got resolved. Would still like to know why.
So would I. Are there logs on hub mesh?
Not really, not even in the engineering logs. It would produce too much chatter and make troubleshooting everything else way more difficult.
I don't know why exactly this happens, but I know which part of hub mesh can be modified to make discovery more robust. There'll be a change in 2.3.1 for that.
Hub mesh stopped working again today 
Tried switching to TCP, and still doesnāt work. Without logs this is very frustrating.
I think the only change has been router firmware in several days.
After giving up and leaving it on TCP. About 36 hours later it just starts working. Not sure what to do, but leave it until the next failure.
Those instructions are useless, there is no longer a Hub Mesh button under Devices and both my hubs see each other but can not access any devices.
Point taken, the Hub Mesh interface has been completely reworked and the documentation has not been updated accordingly. It is in the works, though so the new document will be made public soon. Thanks for pointing this out.
I had to roll back to 2.3.1 to restore the "Share local devices:" menu. I can now see and use them in the second hub but it can still not see the two new devices I added to the second hub. 2.3.1 already has the Mesh Hub toggle as well so that is the same. That toggle also work fine under 2.3.1 but 2.3.2 did not create links when I tried using the toggle only even though it would have taken a long time compared to just using the removed "Share local devices:" menu which is fast is easy. I hope if I update then they will appear on the first hub. 2.3.2 refuses to talk to to 2.3,1 but 2.3.1 works fine linking devices over to 2.3.2.
Of course, neither have that Hub Mesh Button that goes to the menu still in 2.3.1 but removed from 2.3.2 and I have no instruction for how to link under 2.3.2 but have spent days working with it I finally found that funny symbol in front of the item may move it into the shared group? Maybe? Then they should appear under the "Available on" list? Maybe? I am just randomly trying stuff at this point.
The docs were updated shortly after your post. There may still be some issues enabling Hub Mesh on new 2.3.2 hubs that didn't previously have it enabled (I wasn't able to, at least...), which may be part of your other problem, though I don't think I've seen any reports of it not working at all beyond that.
Updating both to 2.3.2,130, pray for me
Oddly, some zigbee devices that are only sensors work on both hubs without having to Hub Mesh once I added two zigbee devices to the new hub. Not true for Z-wave but I wonder if that would change if I added one t the new hub. I have lot of zigbee temperature sensors that report fine but my contact sensors are Z-Wave Plus and are not working unless I Hub Mesh them into the newer hub. I was getting slow responses on my contact sensors which was why I added the new hub. I want those events on it. I do some complicated stuff, one sensor may have a lot of apps that check conditions like time of day, length open, current state, etc. and react differently.
It is NOW working!!!
Download the Hubitat app
