Is the user in your ldap address book or local address book? If ldap, then it's possible that your users don't have the required freebusy url. To verify, login to the console and type slapcat. If none of your users have anything in the calFBURL section, that that's your issue. To fix, goto server-manager, users, modify a user, don't change anything, just click save. This should save the freebusy info.
By default users don't have this. You only get this option when you add kronolith so you need to prep each user. There is a box that you can manually add this info to, but it's also saved when you modify a user.
If the user is in your local address book, then you have to give them a freebusy url that can be used.
John