I have a similar issue to visumbra, though basically in reverse. Synchronisation from Android to Windows 10 per Syncthing works, but it doesn't the other way around.
I tested whether Syncthing can synchronize in both directions, and it can.
What I noticed is that Joplin displays the following error message every time I try to sync:"Error: Got metadata for path but could not fetch content: bd7e689960e0419bb9db8c644b23975e.md". I don't guarantee that I typed that filename correctly, but that was probably a pointless exercise anyway! 
I'm at a loss now what I should do about this error, so some pointers in the right direction would be very appreciated!