My IMs are not visible?

It appears that recently, after years without problems, my IMs are not visible to the contacts to whom they are sent. I have been able to chat successfully in a chat room with a contact, but my private messages to that contact are not seen. I am able to see the private messages sent to me, but it seems that my replys do not get through. I am using windows 7..............any ideas on how to fix this?