I think I’ve found the issue, it looks identical to the issue reported here.
No automatic sync
I’m running DAVdroid 0.8.4.1 from F-droid on Android 5.1.1 on a Sony Xperia Z3 (stock ROM). DAVdroid will manually sync with Fastmail’s caldav server but it will not automatically sync.
The account settings all say automatic sync is enabled but it doesn’t matter if I set it to 5 minutes or 4 hours, it always shows the last sync as the last manual sync and the calendar is never updated. It also does a single one-time sync if the phone is rebooted and then never syncs again (unless you initiate a manual sync). This really limits the usefulness of DAVdroid.
Are you sure sync is enabled system-wide?
Do you sync on mobile network or WiFi? Are you sure that “Restrict background data” or similar settings are not active?
Do other sync providers (email, …) sync automatically?
Thanks for your reply. The Sony Z3 has a battery saving mode and it turns out it disables sync whenever the screen is off. I’ve not had a problem with other apps, sync, etc, but it stops DAVdroid from syncing except when the screen is on. So that seems to be what was causing the sync failure.
I see. However - all the other apps then don’t respect the battery saving mode where background syncs should not be running to save the battery…