4 MACs one calendar and contact Sync issue

[3tpro]

We have a user that has 4 macs and wants to sync all 4 to the same calendar and contacts. We have installed Zimlet for mac version 5.0.2201 on all 4 machines. We are experiencing the following issues.

Items that are placed on the calendar do not replicate to other computers or the webmail interface. We have the zimlet configured to sync every 5 minutes. After the 5 minutes has passed and we see the auto sync progress bar we get nothing on the other machines (after waiting another 5 minutes). We do see the new information in the webmail/calendar interface for this user. If we then press the "Sync Now" button we get the calendar items on the other machines. We have tried the following without any luck.

1. Change calendar to "Selected" and pick the one calendar
2. Change calendar to "CalDAV" and pick the one calendar

Nothing seems to make this work except to manually press the SYNC button

Systems: mac are OSX Version 10.5.2

Any help on this matter would be appreciated.