@morrow without logs this will be hard to track down. can you ask your administrator to open up a support case for this?
If after the creation of the user profiles in Outlook the public store is created, this store will not be visible to the users whose Outlook profile have been previously created. Without access to the public store, the freebusy times will not be available either. The same problem can occur when migrating from old pst files, where the same profile is being used with Zarafa. The profile will reuse the old data which do not contain the public store. In order to make the freebusy data available again, new profiles should be created.