If the user have a different mailserver or a different maildomain in the users person document, than the other people, the user will not show op in calconn and of course the user vill not have any entry in Group Calendar.

On the Global Option document in Calconn, you specify what mailserver Calconn should gather information from. One server or many servers. 
That or those mail server(s) can have the same mail domain, but in some case thay have different maildomains.

Therefore you should check if the user is 100% identical with persons that will show up in Calconn and in the Group Calendar.

In NAB - names.nsf -  please find the fields Mail Server and Mail Domain and verifie that theese 2 fields holds the same information for the person NOT in Calconn and for those listet in Calconn.