Read Only Calendars are no longer showing up in the Delegates listing

Earlier this week I had a Calendar (the boss's calendar), disappear from all the users that had Read Only access. All of the users that had Read/Write access can still see the Calendar. A couple of days later, I had another Read Only Calendar disappear from users iCal.

Our Calendar data is stored on a Snow Leopard Server based XServe using Apple's built in iCal Server. The calendars that are diappearing no longer appear in the Delegation tab of the Accounts section on the User's individual iCal app preferences. If I move the user to the Read/Write group, the calendars re-appear.

Any suggestions?