connecting ...

I have six computers connected with Teamviewer. Occasionally, I try to connect from A to B, and just get a "Connecting ..." message, that never gets to authentication. I can connect to all the others just fine. TV is running and I am online on B. In fact, I can establish TV connections from B to other machines with no problem. Just not to A. So the A to B Teamviewer link is hosed. I've tried restarting TV on both machines. No joy. What's going on?