Tray icon invisible

Running Vista SP2 and Multiplicity 1.22

Now and then, and I can't figure out the pattern, the tray icon on my controlling computer goes invisible when my other computer is disconnected from the network. The space is still there in the tray, and you the tooltip pops up when you hover over the space.  But I can't get the right-click menu.  I have to run the executable to get the configuration panel.

anyone know why this happens?  thanks!