Changes to the roster may occur for different reasons. You might change the roster yourself. Another application, logged in to the same account as you, may change the roster. And those that you have presence subscriptions to may change the state of the subscription. Your application therefore needs to respond to changes in the roster, to maintain its state of operation. There are three events you need to listen to: when roster items are added, updated, or deleted:

this.xmppClient.OnRosterItemAdded += 
   XmppClient_OnRosterItemAdded; 
this.xmppClient.OnRosterItemUpdated += 
   XmppClient_OnRosterItemUpdated; 
this.xmppClient.OnRosterItemRemoved += 
   XmppClient_OnRosterItemRemoved;