My previous situation:
Windows 7 laptop Lubuntu laptop Android phone
All devices were added to each other as introducers with various shared folders. Everything worked fine.
Then I recently did a fresh install (not an upgrade) to Windows 10. Since then syncthing on the Windows 10 device rarely connects to any of my other devices. I have seen it connect very intermittently, for unknown reasons.
I’ve done numerous experiments, but this was the most extreme:
Reinstalled syncthing on Windows 10 and Android. Didn’t reinstall it on Lubuntu, but removed all of the old folders and devices to get it close to a fresh state.
Using the ID UPC on Lubuntu, I added Lubuntu to Android, as an introducer. Lubuntu saw Android almost immediately. Shared a folder and everything is working.
On Windows 10, turned Windows Firewall OFF, to eliminate any question of firewall issues.
Using the ID UPC on Windows 10, I added Windows 10 to Android, as an introducer. Currently 3-hour later and it hasn’t connected.
This was all on my local WiFi. On all devices Global Discovery is reading 1/2, which I assume means Windows 10 is talking to my router just fine. I tried this with both 32-bit and 64-bit syncthing. As far as I know, there is nothing special about the Windows 10 machine: no other firewalls and no fancy networking set up. Just a normal install.
I’ve also tried adding Android and Lubuntu to Windows 10 manually and that didn’t work either.