Since I only have Linux machines, no Windows, that could not be the cause in my case. I had that happen several times, and it always were small files with size < 512 byte. And one time I had a new file that did not trigger a sync, even after a long time (the docu says that every 10 minutes a sync is initiated even when ther was no inotify event). The unreliability of btsync is so frustrating that I switched to Owncloud sync. So I cannot further investigate on this topic.