Hi Patty,
Welcome to the Apple Discussions
This sounds like a Buddy is either Blocking you or you, them.
Go to the iChat View menu.
Select Show Off Line Buddies
Does the Buddy now appear in pale grey text ?
If this is the case go to iChat > Preferences > Accounts
Select the Jabber account in the list on the left
Select Security tab on the right
Make sure Allow All or Allow Buddy list is selected.
In the Address Book check the spelling of the Jabber name.
In iChat if you can see the Buddy's name check it is spelt correctly.
Also highlight the Buddy and use the Apple and i keys together (Get Info)
Select the Account name in the drop down.
Check the screen Jabber name here as well.
8:23 PM Thursday; July 20, 2006