Please check if the automatic synchronisation is activated in the setup CalDAV connection and a proper interval is set.
Open App -> Synchronisation -> Click on respective account -> Select calendars and continue -> See if automatic synchronisation is activated.

The automatic synchronisation does only work if you enable automatic synchronisation system wide.
Depending on your android version, there are different ways to enable it:

• Settings -> Accounts -> Automatic synchronisation
• Open the “Contacts” app, Menue -> Accounts -> Auto-sync app data
• Pull down notificationsbar -> Sync

Now ALL accounts will be synced. If you don’t like automatic synchronisation for all application, you have to deactivate this for each single account:

• Settings -> Accounts -> Choose account to disable and deactivate synchronisation

Posted in: CalDAV