Send console message error?

I have three desktop computers all running windows xp home. 1 retail, and two
OEM, and a laptop running xp oem. all are connected via Belkin ADSL modem
with high speed mode wireless router. and all four have wirless network
cards. all are setup and running on the internet. I can see two computers and
the laptop on my computer but not the other one. I have tried to send a
message via the computer management window: ie; send console message. but
when I send a message it comes back with an error( not all recipients
received the message: this could be due to an invalid computer name, or
because the recipient computer is unavailable.
Check each recipent computer name and then re-send.


what am I doing wrong.

Cheers Brian