Can send messages, but can't reply to messages

My client uses VCM, a messenger program.  I have 7 workstations, not all have this installed yet, however, some of my users are having problems utilizing this program.  Some users can send a message, and it's received, but the recipient can't reply.  A pop-box comes up stating "at least one user must be selected".
The recipient is shown on the sender's computer as online, but the recipient's computer has everybody as offline, which they aren't.
What would be the first step in troubleshooting this?
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Eric AKA NetminderCommented:
Just a guess: check the recipient's settings. If they're set to "hide", then that could be doing it.

The other issue COULD be a firewall setting.

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