It will sync, it takes very long time. I had the same problem, after the initial sync it works flawlessly.
Here is the response I got from the dev:
Yes, the app should be working just fine.
You’re right in that syncing takes a while (up to a few hours).
I recommend turning off the display sleep, let the app stay in the foreground and a strong WiFi. Then it syncs.
Von meinem iPhone gesendet