Sometimes the synchronization between Dive Organizer and iDiversDiary could be affected by an issue with port communication.

Taking a look at this KB on Apple site

https://support.apple.com/en-us/HT202944

It follows that the network firewall must be configurated to allow the traffic through the following ports:

1900 UDP
5350 UDP
5351 UDP
5353 UDP

Please refer to the manual of your router/firewall or an helpdesk service to help you to add the more suitable rules.

Other useful articles

How to troubleshoot sync activities