I enabled debug logging which seemed to help stop files from getting stuck, however I've discovered that it's certain files in particular that become stuck syncing.
InDesign files, and Adobe X D files. Both look as though they create temporary files which cause Sync to become stuck, looking at the logs it looks as though Sync hashes the temp file, tries to sync, but the file changes, so therefore the two files don't match, and the sync fails.
I've added *.idlk (InDesign lock files) and *.~tmp (X D Temp files) to the IgnoreList files in the .sync folders on all the machines and so far things seem to be back to normal.
https://help.resilio.com/hc/en-us/articles/205458165-Ignoring-files-in-Sync-Ignore-List-
Hope this helps.